I think his character was upset over the guilt of lying to his student/fellow tenant about his concussion being fatal, which in turn brought about his suicide by jumping off the building. He lied as a way of revenge because the student was on the higher floors and acted pompously towards his teacher.

They wanted to go back in the buildings because they were drawn to the perversion of the social collapse. In their mind, why go to work, get stuck in traffic, and pay bills? They wanted to rewrite the social rules, as seen by those in the lower floors climbing to the top. In their mind, they wanted to establish their…

oh, I see. Yes, he does seem much older :D

They're 16 years apart in age. RDJ is 51. Hiddleston is 35.