Except Killian wasn't after vengeance. In fact, at the start of the movie, he isn't after Stark at all. And, in fact, just the opposite. He comes to Stark Industries to specifically meet with Pepper Potts, since he has personal and business ties to her, to ask her to consider funding Extremis. You need to watch the movie more carefully. Especially the scenes involving Killian. You don't think glowing eyes, glowing skin, the ability to breathe fire and hurl massive multi-ton objects, to emerge unscathed from huge explosions, the ability to grow back severed limbs instantly, the ability to command an army of similarly powered soldiers, and having The Leader of the Free World in his back pocket is menacing....? ....Really???