Can you explain this phrase "You are dead to us. We hope you fail", please? It looks like sarcasm. Is it OK for colleague farewell? Is "We hope you fail" a good wish and why?
Are you familiar with the concept of a "roast"? I didn't know whether this was culture-specific, and I see now that, according to Wikipedia, it's an American thing. It's a tradition of insulting people or wishing them ill as a way of honoring them or wishing them good luck. The TV channel "Comedy Central" sometimes hosts a "celebrity roast" where someone from the entertainment industry is "honored" by colleague after colleague from throughout the person's career taking them microphone and saying the most horrible things about the honoree. Years ago, the entertainer Dean Martin was known for hosting these.
The implication is that the honoree knows that they are so well loved that it is safe to take the insults all in a spirit of fun and knowing that nobody means what they've said seriously.