Not having empathy for someone who didn’t extend it to others is a normal human reaction. You’re not obligated to manufacture sorrow and empathy for anyone.
Should it have happened? No. Does everyone have to feel bad? Also no.
However, I do feel terrible for his wife and kids.
Compassion has to have its limits. For example, there's a reason nobody joked about Robin Williams death. If you spend your life doing nothing but bringing the world joy and spreading love, you will have your death praised by all.
But if you dedicate your life to furthering the divide of Americans and spreading hatred, you're naturally going to get a lot of people who don't care to mourn your death. Again, compassion has its limits.
I personally believe that nothing justifies it.
With that being said, Charlie Kirk himself would justify it, had it happened to someone else.