As a lawyer, I feel like the slightest bit of legal knowledge makes this even more baffling. The answer to most questions is "it depends", but the answer about doing anything concerning a witness against you in a trial is "heck no".

There's no logical reason to do this. I think the answer is that no matter how smart he may be in some areas, he has emotions like anyone else which sometimes lead him to do stupid things. I think many of his decisions are also rooted in a copious amount of pride which leads him to think he will get away with anything.