He may have thought he was too smart to get caught. Or could talk his way out of an arrest or being found guilty.All the time he was ruining Monique's life before he ended it (and Spencer's too of course), did he not realize that he was doing the exact same thing to himself? He can't be sane.
Or maybe he wants the attention of a trial, wants to present a manifesto about how the world has wronged him, air his imagined grievances.
Or maybe he no longer cared about what would happen to him.
Or didn't think through the consequences due to mental illness or drugs, etc.