Why did we kill Malygos again?

[Dunee]

He was insane, so he had to be taken care of.

[Tonyman64]

He was redirecting leylines and this would have destroyed Azeroth. No conspiracy by Bliz, no silly 'just doing it for selfish reasons' thing going on; it was just a simple prevention of the destruction of the world.

[Slavemaker]

I think you meant "Why did we kill Malygos, again?" I read it as if we killed him twice.... Blah.

Basically which I am sure was explained already, he was going insane, quite literally insane, he thought that anybody who was not of the Blue Dragonflight was to be destroyed for "abusing magic", or something along those lines.

[ELYPOP]

I think that Venziir summed it up in the best possible way.

His initial insanity kicked in when 95% of his flight was obliterated by Deathwing 10,000 years ago, and when that was "cured" by the Netherdrakes from Outland (Brought with Tyrigosa, his daughter.), he declared war on all of the magic users of Azeroth. So that the Kirin Tor could spearhead the mortal's side of this war, Dalaran was relocated to Northrend. Because of the massive loss of life that was going to take place, Alexstrasza stepped in.

Not only was he going to decimate all of the mortal spellcasters upon Azeroth, but he was also redirecting the planet's Ley Lines (Magical web sort of thing, think of multiple rivers.) towards his base at Coldarra. He was a little less crazy than what he was for the last 10,000 years, but still crazy enough to pose a threat. That's why we killed him.