I know I have said this before myself, that we cannot possibly understand why killers do what they do because their head is messed up and it will NEVER make sense.
But seriously, this is one case that has me going WHY?! What sense did this make? You have two girlfriends, you get one pregnant, she threatens to reveal this to your other girlfriend....and in your brain, you have concluded the *best* way to fix all this to SHOOT her?!?! What??? Like not even in the moment angry.....he really thought about this and had it preplanned and really thought this was the best way? Okay.....
The only thing I can think, is that these people REALLY don't understand how stupid/amateur they are. They honestly think they are smarter than everyone else and convince themselves they will get away with it. It would almost be laughable if it wasn't so horrendous.