Having finally caught up with it the other day, may I cynically point out that this is the CW - former home of Buffy the Vampire Slayer, Smallville, and current home of The Vampire Diaries? Angst (particularly teenage and romantic) is practically a founding principle with this network.
And didn't react fast enough to do it a second time - which makes sense, given that Ivo had less physical effort to make to shoot Shado than Oliver did to block another shot. Had the situation been reversed, the only thing different would've been the identity of the dead girl, and I'm willing to bet that Ivo chose Sarah first simply because he was certain Oliver was attached to her, knowing her better himself. Maybe he even wanted to spare Sarah out of some feeling of attachment of his own.
So no, Oliver didn't choose, and it wasn't his fault - but he's not experienced enough yet to understand the difference between firing a bullet and failing to step in front of one, and Slade's in no condition to listen to reason.
. . . Which, on reflection really speaks to how badly Mirakuru screws you up. Given Slade's profession, he'd have been trained to make exactly this kind of call, under exactly these kinds of pressures. That he's twisted up this badly, then, means that either he was a lot more attached to Shado than we ever realised - and I'm with Forumghost on the "sudden" aspect of all this - or the drug is just that powerful. Which, while in character (to a degree) for Deathstroke the Terminator . . .
Honestly, that's scary.
Speaking as somebody with a similar family experience (which is all I'll say), I find forgiving Moira harder, myself. It's not just the affair, or the cheating - it's the lie. It's the kind of effects the lie can have. For example, it means that Thea's medical history is wrong; that she - or her children - could be susceptible to conditions that they'd never be aware of until it's too late, because they're not factoring in the Merlyn part of the equation. And in the immediate situation - Merlyn is alive and an active threat to Thea, every minute. And she has no idea whatsoever.But then, Oliver had a tendency to over-react to things. His mother for example. Man, that plot makes Oliver seem like a dick. I mean, it's not like she slept with Merlin after she knew what kind of a monster he is. And if it's the cheating... Ollie, people in glass houses shouldn't throw stones.
. . . I can understand not telling her until she's of an age to handle it. I can understand being reluctant to tell her, given Merlyn's actions. But never telling her at all . . .? No. I find Oliver's attitude perfectly justified - though as previously admitted, I have a bias.