There is no dearth of competition for what kills joy the most. What about envy? Is there anything that belittles all your reasons for feeling happy than to compare it to what the other guy possesses and you do not? What about lack of self-confidence and a sense of inferiority? Is there anything that kills all possibility of joy in company or at work than the feeling that everyone is looking down on you? What about...you get the picture. Joy is a fragile flower, easily destroyed by a multitude of things.
Then why categorise ONE thing as the biggest Killjoy? I'd say it's just because philosophers, like poets, are given to exaggeration to drive home their point. Whether Tiru is just doing this here or not, you judge for yourself.
Nagaiyum uvagaiyum kollum sinaththin pagaiyum ulavo pira - Tirukkural
Is there a worse enemy than anger which murders your smiles and your joy? - Loose Translation
True, Tiru only categorises it as the worst enemy, not the worst killjoy. But IF anything else seemed to him to be a worser killjoy than anger, would that not automatically become a worser enemy?
But, really, what do philosophers have against the poor short-tempered chaps in general? I mean, yeah, I would prefer those around me to be even-tempered but, sometimes, you actually do feel more comfort with a short-tempered chap because, with him, you generally know where you stand. The even-tempered guy may react the same whether he liked what you did or not. (NOTE the 'may'. There ARE even-tempered guys who can tell you to go to hell and make you look forward to the trip, like diplomats are supposed to be able to do. There are others who just go along with you in order not to rock the boat.) But the short-tempered guy probably cannot help getting angry, he can only control how he expresses it.
But, then, I think Tiru does not really mean the guy who screams 'F*@#' when he stubs his toe. Tiru does not mean these small flashes of irritation that last no longer than a few minutes. Anger of the sort that kills your joy is the thing that roots itself in your head and refuses to let go; the sort which flares up over and over again every time you remember the incident or that person; the sort that MAKES you remember that thing over and over, making ANY joyous thought an ephemeral bubble which soon bursts in the flame of remembered anger.
True, the short temper which flares and subsides within seconds can also keep away people; but, sooner or later, most people know to look beyond the ranting and see you for the character you are. That deep-seated anger, though, may not even be visible to most and, yet, will kill all your joys effectively. (AND, yes, those insecurities, those envies, which are also killjoys, root themselves right there in the mind. In the longevity of your memory and in the manner in which you select memories to revive and hold close, the emotions that you continuously refresh.)
It is in the mind that we end up discarding our friends - compassion, love, affection etc - and hug closest our enemies - anger, hurt, envy etc. So the biggest killjoy (as also the biggest source of joy) IS your mind!
