What exactly do you define as a "malevolent" companion?
One person's saint is another person's sinner. I find most members of the Chantry to be far more sinister than the average Tevinter magister.
Unsurprisingly, I disagree with your statement that Morrigan is at best a "cartoonish portrayal of evil." She was never written to be evil per se, but rather representative of the conflicts inherent to Thedas.
Which suggestions, specificly, are we talking about here?
- Harness the power of the Anvil? Sure, it's a risk, but it could prove critical in thwarting future Darkspawn invasions.
- Perform the Dark Ritual? She cared for the Warden and did not want to see him or her die. Also, she has an agenda which she is pursuing that neither you nor I are privy to.
- Accept Caladrius' blood ritual? From a purely pragmatic standpoint, it makes sense to me. Anything to increase the power of the Warden.
- Killing Flemeth? Can't argue with that. Morrigan gains her mother's Grimmoire in the process and, at the bare minimum, buys herself sometime before Flemeth attempts to possess her body.
- Aiding the Warden on his quest to defeat the Darkspawn?
Her reasoning seems pretty sound to me.
I didn't mention the anvil because i view that decision as common sense, golems are useful and effective against dark spawn and i would be a fool to destroy such a weapon, i didn't need Morrigan to tell me that.
1.Killing the elves for volatile werewolf's just because.
2.She disapproves if you save Redcliffe which could provide soldiers and supplies to the army. = Valuable Resources.
3. The ritual was Flemeth idea not Morrigans.
4. As for killing Flemeth, that was also common sense, when you find someone is out to posses your body not killing him would be foolish.
and her crowing moment:
Killing the circle of magi just because she didn't like them, and wasting a very valuable Resource.
As for what defines a "malevolent" companion, for me it is a lack of remorse, a will to do what needs to be done, no bellyaching about collateral damage, and most importantly the ability to make intelligent choices whether they involve saving a village or destroying it, all that should matter is how our actions would contribute to achieving our overall goals