I'm genuinely curious because I've seen this kind of knee-jerk response across many avenues (e.g. x is an antisemite, is hitler, is a facist, is a commie, is a trump supporter) and I don't understand it.
In the medieval era most people participating in European civilization would qualify for the modern definition of antisemitism. Should I simply disregard the good works of writers and thinkers from that time period and geography?
If anything I am pro-semite but it's a struggle for me to grasp why the antisemitism of an individual should affect my opinion of their work or ideas. Even though I don't approve of their opinions regarding a people group it simply does not follow that I ought to dismiss everything they have ever said.