1. How could we not kill him? He IS a villan. He is/was the tyranical ruler of Outland and ruled with, well, an iron hoof.

2. Blizzard can HARDILY ruin the lore behind him because they CREATED the lore behind him! If it wasnt for that, the final boss of Burning Crusade could have been, hell, a large, rainbow colored core hound!

3. The most complex character? I beg to differ. I would say The Lich King is, because not only was Arthas Menethil a crusader for Humans, Paladins, Lorderon, and just good 'ol righteousness. He betrayed all of the above stated, slaughtered his mentor, and also fused his soul with an equally dynamic character: Ner'zhul.
He was one of the most popular and important figures in orcish society, hailing as chief Shaman for the Shadowmoon Clan. He was contacted by his dead wife's soul, and told to betray the friendship with the Draenei, systematicly causing the genocide of the fugitives of the Burning Legion. He later figured out this was the demon lord Kil'jaeden, and ceased all actions against the Draenei, but it was too late for him. His apprentice Gul'dan was enticed by Kil'jaeden, took command of the new orcish "Horde", and contenued to slaughter innocents, rip the planet apart witht he help of Medivh (another equally evil character, as he was consumend by Seargas), invade Azeroth, and start a war (or 2).
His body was destroyed by Kil'jaeden, his soul contenuenly tourtured, and then forced to server the demon, and his soul then bound to the armor we see Arthas wearing, which ended up fusing their souls together, serving Kil'jaeden as one, and ultimatly creating the scourage, destroying Lorderon, killing billions and transforming them into their own enemy, taking over of who knows how many factions and species such as the Nerubians, and becoming the reason we fight him in the latest expansion.
Lorewise, the Lich King triumphs Illidan any day.
and lastley,
4. Designated insanity? He betrayed his brother, the entire Nelf people, became half demon, and might just be working for the Burning Leigon. How could he NOT be insane?

I hope Iv made my point.

No Phort, no.
The LICH KING is an amalgamation of two other characters. Sure, independently, they are big stories. But no, they are not complex in the same fashion as Illidan. Ner'zhul is NOT Guldan, he was simply his teacher. Adding that stuff about teaching the guy who lead the Orcish Horde- is like anyone else drawing the argument that Illidan, equally impressive, inspired Malfurion Stormrage, his twin brother, to lead the NightElves to banish the demons from Azeroth, twice... Thats not straight lore, and its just not that kind of a ho-down.

Illidan, was over 10,000 years old. And instrumental in MANY parts of the lore. He was NEVER the bad guy, just as Cowfordflorida posted. He never betrayed his brother, at least not in his own eyes. Reread the story from a less biased position man. If there was EVER a story about how a man could take it all on the chin (no Deathwing pun intended), its Illidan.
If you have no idea what kind of duality Illidan goes about in his plans throughout the entire Warcraft series, you will think that he is simply The Betrayer and simply working for the Burning Legion with punchcard in-hand. That, is the BC lore that was presented to raiders. To make sense of us busting up in his joint and killing him.
If you think you got his story in the bag, check again, because you've probably missed a few things- I have typed this next part out for the last hour while editing it all out so I don't rant any further... But its 3am, I'm not going to do the work for you, go get the books, sit down and read, find out what it means for a Nightelf to become a Demon Hunter and for goodness sake; the guy was cut by the soul stealing blade... And spent the rest of his days awaiting fiery death which could come at any moment, only to be killed by the stalkerish warden of the barrow dens (where he spent 10 thousand years paying for pretty much granting Nightelves immortality) and us, a bunch of ill prepared misfits.

Whereas Arthas died after being strung up like a piñata 1 shot by Tirion Fordring. (Yes I am aware of the technicalities of this)

Do you still want to make the argument that they don't "ruin" their own lore?