See, I think it's like 'I was desperately lonely because the world has shunned me for all my life, then you came and gave my life some meaning and I feel like we shared in each others' loneliness. I'm obsessively in love with you and can't bear the thought of losing you. I also feel like you're my last/only chance to get out of this depressing cellar life and to live as a human being. Without you I have no reason/strength to live. So I'm blackmailing you with the life of the man who's threatning to take you, and all my strength, hopes and dreams away from me forever.'

And, at least in the ALW-musical he probably believed she doesn't love him as she'd taken his mask on stage, and with his revenge strike...you know...

Hence, throoughly understandable motives. And then he redeemed himself by letting them both go.

PS.
However, when he repeats this action in LND but with her child and just to hear her sing once more, well, THAT is unforgivable. Because it is utterly not understandable and he doesn't even apologize for it or redeem himself in any way. The entire thing anyway would be highly unlikely to happen after PotO's end, as in unrealistic. No matter what ALW says, LND will never be canon to me.

Well if you think about it in a real life situation coming to light, we would be like "you fiend! How could you! Monster, lock him away!" And so on...
But the story itself is fantasy, so of course we're on Erik's side naturally (where you'll usually find me) though he does redeem himself by essentialy dying, well letting Christine and Raoul go than dying. And thats the perfect ending. I go as far as wanting to starve Erik of compassion and love because it makes him a more interesting character. It's mean, its terrible I know, but it needs to be done. LND only serves as fanfiction for those who don't like the phantom as a "bad guy" and want him to have a disney ending. (It was disney-esc the mom died!) But I'm not going to harp on that.
Erik redeems himself, and I always forgive him. <3
...except when he's Gerard Bulter, that's unforgivable!

"Well if you think about it in a real life situation coming to light, we would be like "you fiend! How could you! Monster, lock him away!" And so on..."

Actually, you can't really generalize like that. You can't tell how other people would react to a real life case. And remember that if we are to compare in a real-life sense, we must think in a real life case of which we have exactly the same amount of knowledge. That means, we would know what his motives were and that he let them go after all. Therefore most certainly no, WE would not be like that. SOME people might feel entirely differently.

I personally would not call him a fiend and shout "how could you". Because, as said, I understand his motives and he redeemed himself at the end - plus, he didn't harm my loved ones, or a child. Therefore I see no reason for me to not forgive. But this doesn't mean I'd wish him to go free. I'd wish him to be locked into a hospital for the criminally insane, to receive professional help. In the fictional case I of course take the story up somewhat different path.

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Forgivable of course. I hate Raoul and Christine, to me, they could die :)