I can see what you mean but as you mentioned at the end it was "one rotten apple" one small example in comparison to the large percentage of people who do immoral acts and are not vegan. That was the correlation I tried to highlight and in my opinion a few anomalous examples do not affect the overwhelming trend.
But I do accept that within every group there will always be the more radical members, like this vegan activist and I disagree with what he done completely. I do not know the context of this incident but I can imagine the politician was maybe making fun of the animals that have to suffer or not caring about them. If that is the case then I cant agree with the vegan activist for killing him, but I cant agree with the idea that it was completely unjust because the politician has consciously contributed to the suffering and death of millions of animals through his food choices, so to not expect or accept someone to disregard his right to life is hypocritical in my opinion.