They switched writers.

I guess they did some market research between the two testaments

its almost like the whole thing is an amalgam of thousands of texts edited and repurposed across thousands of years by human beings with various motivations.

Because they're completely different gods. The old testament is only a part of christianity because in order to gain some legitimacy for their early church, they decided that their new god must be the same dude as the the god of the people that they were living among.

But in reality, they are very different books, written in very different times, by two very different religious cultures.

Why Hulk can defeat Wolverine in one comic but in the next one gets obliterated by someone weaker?

It's because the Old Testament is actually just the Torah, rearranged and edited to fit the beliefs of what was once a sect of Judaism. That sect branched off when they decided that Jesus Christ was their Messiah, then progressively became more open and split away from the rest of Judaism and became their own religion.

That might be a bit oversimplified, but that's really the gist of it. Jesus made a new covenant with god, which was meant to replace the old one, chronicled in the New Testament; but the old covenant was kept in as background, becoming the Old Testament.

Because it's all fake. Everyone who actually reads it finds way too many inconsistencies.

That's because it underwent some serious transformations across the millennia. Yahweh started as a storm god (basically Thor of Canaanite religion). Back then each nation in the religion had their own patron god and guess which god did the Israelites happen to have? Good old storm god Yahweh.

Over centuries the religion evolved and among Israelites Yahweh slowly took on attributes of other gods, mostly El (the all-father and creator of the universe) and Baal. First the other gods were degraded and monotheism was required, even though other gods were known to exist (you might remember the whole "jealous of other gods shtick" even though the rest of the Bible says there's only one god).

Then the other gods were slowly edited out of the Bible, though some remains persevere (the aforementioned jealousy of other gods, some gods are even mentioned by name). If the gods couldn't be removed because the story wouldn't make sense, they were mostly changed into angels or other mythical beings.

It's pretty funny rereading the Bible with this knowledge, you can clearly recognise which parts were the original Yahweh-the-storm-god and which used to be El-the-actual-creator by how he behaves in the story. When he's all jealous, rageful and angry, it's mostly based on the original Yahweh.

Anyway, that's basically what Old Testament is - a bunch of edits of much older religions. IIRC Yahweh precedes even the Canaanite religion, so it's a really old and grumpy storm god.

Now, New Testament is something else entirely, that was basically just slapped onto Judaism to have some legitimate and widely recognised vessel. Unlike the other edits, it didn't evolve naturally over time, it was just violently slapped onto the Old Testament.

Fun fact: try finding Satan anywhere in the old testament. You won't. Satan has been retrofit on multiple characters, but neither is mentioned directly as Satan, devil or really anything. The most famous one, the snake in the garden? Just a snake (which checks out with older religions where animals had a lot of influence). Then some morons come and say "actually, that snake was the grand adversary." The concept of a grand adversary wasn't really common in older religions, there usually wasn't a Satan-like figure. Compare for example with Greek, Roman or Norse gods.

So, in conclusion, the Bible is a horrible mess of edits that were made so the religion would serve the needs of the time they were introduced in. IIRC the Israelites were having some trouble with their neighbours back when Yahweh got the promotion, so having a strong sense of nationality would really help in keeping the nation together. New Testament is even more obvious because it didn't even really try to fit with the rest. They just tried to retrofit a few things and called it a day.

Well, this got longer than I planned, but I really like the topic and I don't think you can do it justice in two paragraphs. If anyone's interested, do some research, it's honestly fascinating! For example, what's the connection between Dionysus and Yahweh? That would be a homework for ya!

The old testament is essentially Judaism which is an ethnic religion. There is no marketing needed because it is a religion for a specific group of people from a theoretical single lineage. There is no need for God to be accepting or patient since the goal appears to be unify and keep people under control during times of great strife.

Christianity is a universal religion ie. it tries to create new followers. If you're a religion that is trying to grow your following, you need to have a message of openness and acceptance.

Going well beyond my competencies to answer, but I think a lot of it comes down to monotheism changing the nature of god.

Judaism thinks of itself as starting monotheism; and that is largely true. However, the old testament is still littered with vestiges of it's polytheistic origins.

If there are multiple God's, then those God's will come into conflict. That is simply the nature of human storytelling.

Looking at the old Testament, probably the most violent God has been was during exodus. In addition to freeing the Jews, he smite the Egyptians with 10 plagues, among which was the death of all firstborn sons.

For I will pass through the land of Egypt this night, and will smite all the firstborn in the land of Egypt, both man and beast; and against all the gods of Egypt I will execute judgment: I am the LORD. (Exodus 12:12)

Note the polytheistic origins of this story. God is not merely intervening in the Earthly affairs of us lowly humans. The Jewish God is fighting with the Egyptian gods. He does not have the luxury of being nice and good. Even if he wins this fight without resorting to such drastic measures; he still needs to do so to act as a deterrent against other gods acting against him. That is not so much a specific tactical calculation in this case, but the way humans tend to imagine polytheistic gods working (reflective, of course, of the way human conflict tends to work).

It probably doesn't help that Yahweh was the god of War before becoming the only God.

By the time we get to the new testament, the situation is different. Beyond merely declaring that their god is the only God, the early Christians believed it, and had believed it for generations of storytelling. Their view of God had shed the vestiges of polytheism and morphed into what is truly possible under monotheism. God can be good because he lacks a peer rival. There is no narrative reason for God to be mean, because he can simply win any direct confrontation he faces.

We see similar dynamics play out in modern story telling. When we have vastly overpowered characters, the nature of the conflicts they get in us not fights. Perhaps they are trying to mediate between lesser parties. Perhaps they want to get something while respecting the rights and interests in weaker parties. A story where a vastly superior force wants something and just takes it is boring; so we don't tell it.

My take is that it's a reflection of the Israelite people. It's easy to be all fire and brimstone when you can back it up with military force. Suspiciously that all went away after they got conquered...