And it's bad because it makes other people unhappy.

But there are 2 kinds of complaints: legitimate and illegitimate ones.

If a powergamer makes someone else unhappy purely out of spite, with no significant benefit to himself at all - such as fullsaccing everything in sight, then he is making people unhappy, and people have a right to complain about it. There is a small benefit from full-saccing, that is you remove someone from the equation for 2 or 3 RL hours, depending on how fast they regear. But the amount of frustration caused to the other player is just so great that it's not desirable for this option to be exercised.

If a powergamer is just so successful at what he does that everyone else is jealous and thinks he's not 'playing fair' even though he's entirely within the rules, then their complaints are illegitimate.

I wouldn't call playing a goodie healer who uses gate, insects, staff of striking etc to PK powergaming, btw. You could do much, much worse. Decked levelsitting human thief sounds good. They won't even see you coming, and you'll demolish your range with disgusting ease. Elf shield ded Maran at hero is another powergaming example, insofar as any goodie can powergame... Scion conjurer/transmuter/shifter/lich/mummy/AP, but you'll have difficult numbers to fight. Fire giant sword warrior Imperial Warmaster/Emperor too.