Personally, I do joke about depression, having it myself, and honestly this makes me feel better. I joke about a lot of dark or controversial things within my friend group, but the key word is that this is my friend group. However, I only joke about it with people who are also comfortable with it, and most of the jokes are just aimed to make fun of myself instead of other people. I would never go to a random person like that and just tell someone to "kill yourself". There's a difference between making dark jokes in your close circle of friends and telling some random person with depression to die.
I don't get offended by jokes like this, even by strangers, but I can see how some would, and just telling someone to kill themself isn't really humor. There's a huge difference between jokes between friends and just harassment.
Just remember that everyone has a different sense of humor, but the situation the humor is used in is what makes it acceptable or not. In your situation, that's definitely not okay, as that's a random person who has no idea what you're like or what you're comfortable with. He most definitely aimed to harass you and those people, not to make a joke but perhaps didn't know where it was appropriate to make them.
Basically, to sum up my ramble of a point: dark humor and joking about stuff like depression is OK as long as you're within your own group of friends who are comfortable with it and do not feel uncomfortable with this type of humor. Dark humor intended to hurt someone and used with random strangers is NOT OK.
To make it short and sweet,
This is a world that we're growing up in.
Some people have a weird , dark sense of humor. What you saw as dark and what others saw as dark, may not have been dark to them. They might've actually associated themselves with people who joked about the same thing and had no issue with it until they used it outside their known audience.
I believe the part that had them overstepped the line was Go Kill Yourself Since that's a very powerful thing to say. Suicide is not really something that should be taken lightly as it could affect people differently depending on the impact of that / those words.
But yes, I agree.... There are some lovely floofs that are willing to help out or even defend one another in this 'cruel' place we call home.
This is just..horrendous. People like this really sadden me. Can't believe someone would just get a kick out of making such lighthearted jokes about it...