There can only be four reasons Trump defends murderous Russia:
1. Business ties keeps him defending Russia no matter what they do.
2. He' being blackmailed.
3. He doesn't understand what he's saying.
4. He agrees with murder, poison and jailing political opponents.
It's either one of those or a combination. There are no other options.
It's just his personality disorder. Everything is personal with him, including perceived slights. If you disagree, you have slighted him. He defends himself that way because it is instinctive, part of his character. No one should be surprised. I highly doubt if he is in cahoots with them or under any kind of obligation. People keep needling him about his comments, many moons ago, that he admires Putin. The more you needle him, the more the defensiveness comes out.
As for the OP's question, he isn't a debater or a politician and that wasn't his training. He's been the one in charge, answerable to no one else. He doesn't have that ability to come up with a reply on the spur of the moment. And at his age, I doubt if he's going to learn.