If it's a genuine accident, then what does a harsher punishment achieve? People make mistakes. Some people are unfortunate enough that their mistake has grave consequences - doesn't mean they're any more to blame, just unluckier.
Texting and driving is a good example. Every single person who drives has done it at least once in the past and everyone who claims they haven't is lying. What's the difference in the intent of a person like this and one who does end up killing someone while doing so? None. The others simply were luckier. The unlucky ones are not morally more guilty than the others.