Nobody has said people aren’t allowed to hold views that are inherently self-contradictory and illogical and others, therefore, find objectionable. People can privately pursue whatever kind of values they wish provided they don’t abuse others and comply with the law. There are no thought police as far as I’m aware.
The issue arises when people egregiously try to force their extreme and harmful ideologies into public discourse in an attempt to normalise them, or try to change public policy and law to fit their ideology with the intention to try to force compliance with their views onto wider society because they want everyone comply with their own personal beliefs and wish to constrain how others are allowed to live to fit their worldview. Mr Kirk’s political activities had this very clear and expressly stated motivation.
Rights have to be constrained to the extent that everyone in society has the ability to have those rights and exercise them, otherwise they disappear entirely. This is what Mr Kirk appeared not to grasp (again, being charitable here. It seems more likely, in honesty, that he did grasp this but pretended he did not because he was trying to manipulate public opinion).
Once someone starts advocating for the freedoms of others to be constrained beyond the above boundary which is necessary for those freedoms to exist at all, and instead advocating that these freedoms should exist for one group of people but not another simply because it fits with their ideology (with such ideologies being inherently logically inconsistent by definition) or campaigning to attempt to achieve consensus for subjugation of part of the population because “I think this book from 2000 years ago says it’s what should happen” - as Mr Kirk did - then this is a problem for society and absolutely should be called out during the person’s life (as it was in his case) and after their death (as is being done now).
Nobody should have killed him. Nobody should be killing anybody his murderer has been arrested so justice - such as it is in the US - for that crime will take its course.
However, it’s quite clear he was an unpleasant, irrational and inadequate human being in many respects and it appears that he was intent on causing a great deal of harm to many other people so it’s perfectly legitimate for people to continue to point out this fact despite the fact that he has died.