Bruh. Too soon, man.

So this line of reasoning is a trap. People do dumb ass shit all the time for emotional reasons, including criminals. In fact, people doing dumb ass shit is so reliable that police investigative procedures are almost designed around it.

I'm not saying this is the case, but it is very possible that if this person purchased the rifle legally, they believe that the purchase will be traceable back to them and that they will almost certainly be caught. So they could have acted with the mindset that they would be throwing away their own life.

easier to track

Sounds like a feature, not a bug. What better way to stage a false flag operation?

There is no tracking danger from engravings, other than establishing a message, and then seeing if future suspect might agree with the message.

In this case, it's more likely that a fascist loving employee of fascism may want to project more ammunition for fascism.

The rifle is bolt action. The spent casing was still chambered.

I do find it interesting that DOJ lawyers have been seeking the death penalty for Mangione, and linchpin of their case is that he inspired other shooters – something that until recently was openly laughable.
Adding that the story of the supposed writing on the bullets is based on a memo that circulated at the FBI and it’s unknown if that memo was based on accurate information or just the figment of someone’s imagination to create a narrative that people will now just accept as fact.