He sent this tweet two hours after the incident. No one knew at the time what were the motivations behind it. All they knew at the time was that CEO of subsidary headquartered in Minnesota was shot (by accident or otherwise) and that was it. We later learned the rest.

What were the gun related deaths that weren't tweeted about?

By all means I don't feel sorry for this guy, I agree with the sentiment, but I don't want politicians to be the ones asking to murder someone. I want them to regulate, so killing people this way is not possible.