Depends what kind of laughter. If it was because you enjoy hurting people or think it was actually funny then that was a pretty shitty thing to do.

But there is also nervous laughter and it's not always with malicious intent.

I can't ever imagine laughing when telling someone about someones death though. I had to tell my sister about my dad's death and it's a completely heartbreaking experience. True you and your friend probably don't know Robin Williams personally so it's not the same, but he has been pretty impactful to a lot of people and their childhood.