Because trannies are predominately leftists, and this is just how leftists operate. It's (part of) the same reason you see any group that skews on average toward the left take and abuse positions of power.

These people either want power for power's sake, or actually end up believing their own narrative and are trying to Make the World a Better Place™. They're some combination of utopianists and vile self-hating degenerates who want revenge. Both of these groups generally have an 'ends justify the means' mentality. Either they want to punish, or they want to save people and Do Good.

I guess you could say a lot of it comes down to narcissism, too. They suffer from Main Character Syndrome in a big way. Which again goes back to the ends justifying the means, too. They're...Doing Something, and that's important.