Motive is situationally relevant in pretty much all countries. Motive plays a particularly significant role in trying to prove someone may have done something, be it murder or theft.

In this situation, the motive is less important. We know the guy did the thing. He says he did it. He claimed he did it because x, and the court ruled that x wasn’t supported by any other witnesses and was out of character for the person accused of x, and therefore likely did not happen. That is the end of the relevance of the Motive to this situation.

Even in the case that the Motive had been relevant, there is no justification legally for what he did. “This guy said x. I dont like x. I hated the guy that said x, so I stabbed him.” is not a legally recognized justification for self defense.