I'd say she became more twisted after Irene's death because Irene was an emotional anchor for Raven and the only person she completely trusted. Without Destiny to guide her and stabilise her, Raven had no one to rein in her more insane plans, no one to say "This is foolish and will achieve you nothing".

I'd say Rogue was, more or less, useless to Raven when she went to the X-Men, her powers were completely out of control, and destroying her sanity. Yes, it sort of backfired when Rogue achieved her independence, but it was a calculated risk, and this time, Mystique lose.