I agree, it is insensitive to laugh at someone when they say they're suicidal. But in your coworkers' defence, sometimes people laugh when they are uncomfortable or unsure of themselves, and when someone tells them that they're suicidal, most people have absolutely no idea what to do with that information. I'm not saying that laughing is appropriate at all, I'm just trying to point out that they might not be laughing at you so much as in reaction to their own insecurity with the mention of suicide. A lot of peope have difficulties with topics like death, suicide or illness, so when they laugh, it's probably not personal. And if it is personal ... well, they are indecent and insensitive and should be overlooked as completely ignorant.

Lots of hugs.