The stuff that happened with Ultron? You can't lay that at Loki's feet. Might be easy, I know, and believe me when I say I'm tempted, but Tony decided on his own to make Ultron. Without telling the rest of us.
When you've got skills like that, or powers like yours, you don't get to just dive in and tell yourself, "I'm doing the right thing," and nothing's your fault if it goes wrong when you didn't expect it to.