How can someone be so ignorant to think that is funny?
You're talking about someone's personal opinions (is this funny?), not facts (1+1=2 with common math assumptions).
Saying other people are ignorant because their opinions differ from your shows a level of intolerance that seems dishearteningly similar to some of the purported motivations of the attacker in the incident you mentioned. ("I don't like gay people. I feel they should be punished.") If you truly feel sad about other people making such jokes, there's nothing wrong with that. But consider how you being intolerant will make other people sad.
Sort of wondering, do you also get mad when people mention Malzahar spreading Space AIDS? Because HIV/AIDS has killed like 25 million people. Do you get mad when people joke about "feeding like a homeless shelter"? Because something like 7 million people die every year due to malnutrition and starvation. And starving to death over a period of years sounds a lot more pitiable to me than getting shot and dying in a day. (Again, that's just my personal opinion. Whether you agree that millions of people dying is worse than 50 people dying is up to you.)