Its an incredibly difficult one. From the limited information in that article, The fact he chased him and stabbed him 49 times for me goes well beyond self defense. And whilst the autopsy showed that the victim was in a state of arousal before he died, doesn't prove beyond reasonable doubt he was trying to rape him. Even the judge said "on balance" so there is a judgement call there.
Manslaughter seems the right offence given the claims, but I can't get past the frenziedness of the attack.
The issue is self defence needs you to have only done what is needed to protect yourself and stabbing someone 49 times is massive overkill (no pun intended). So yeah manslaughter is the appropriate action.