Many people think that someone who does something as seemingly crazy as what Ted Kaczynski did must surely be insane. Therefore, they think, he must not be competent to stand trial. However, this is really not the case. It is very unusual for someone to be found incompetent to stand trial.
The only issue in questions for competency are whether the defendant is rational and can communicate with his lawyers about the trial and whether he understands the charges and the consequences of being found guilty. There was no question that Kaczynski was able to do all of these things. He may have seemed crazy, but he was clearly able to think and to reason and he clearly understood what was going on. Therefore, he was clearly competent to stand trial.