As a child, I always wondered how one could love another despite the terribly things the other was doing. I also couldn't understand truly loving someone and caring for someone, but being unforgiving about some past event... Yet, not hesitating to help when they're in dire need of it, and then going back to refusing to seeing and/or being with them after helping. The event explained in the link above isn't what made me understand it, but it made me remember the days when I couldn't wrap my mind around it. The word "love" has lost it's true meaning and passion when it's said now-a-days. Hundreds of years ago, "I love you" was one of the most powerful phrases that could be uttered.
When you realize you love someone with your all, then you are subconsciously aware of everything they are conscientiously capable of, understand what they're capable of, and you accept it, from the best of things to the most terrible. Now, you may believe that they would refrain from hurting you because you also truly believe they care about you. You love them for everything that makes up who they are and for what enables them to love you back.
If one day an act against you, your relationship, or something equally terrible, is committed, it may very well be excruciatingly painful for you. But you personally know you already accepted their capability of doing so, even if you also believed they wouldn't hurt you.
True love is being able to care for that person, help them when they're in need, and only wish their decisions (whether made with you or apart from you) brings them the best (even if you maybe hope they could feel the pain they're putting you though). You might not agree with what they did, and you might personally have a terrible time forgiving that person, if you even are able to forgive them, but it does not change the fact that you love them, wish them well, and will help them get through anything you personally deem is too tough to get through alone.
If you dont understand the moral capabilities of someone, you wish terrible things upon someone who opposed you, you couldn't care any less about how they are, or you ignore them gleefully when they come to you for help with something thats beyond their capacity to accomplish.. Then I have to say you don't truly love them. Maybe you were infatuated with them before you were hurt, or maybe it was just a crush.. But true love knows no hate.
---Edit---
Q: ...But what if you can forgive them?
A: I didn't say true love is being unforgiving but full of care. If you can forgive them, then go you. Some people have a terrible time with forgiveness, some dont.
Q: You can still care for them even if they kill your family, or are a mass murderer?!
A: I said if you truly love someone, then you are aware of what they're capable of (morally, in this case), understand what they're capable of, and accept it. Is your significant other really morally capable of committing mass murder? And, if you love them, and they are capable of it, then it's probably not a big deal to you. (Like Eva was to Hitler, for example)
Q: What if I refuse to help them because all they're doing is taking advantage of my willingness to help.
A: My exact words: "...help them when they're in need.." If you believe they're just taking advantage of your willingness to help, then they're not truly in need.
Q: What if you believe you love someone, but they're hiding something about themselves (like being a mass murderer from question 2)
A: There's always an exception to the rule. If you aren't aware/cant understand/dont accept something they're hiding or lying about, and you find out, then you were in love with the person crafted from lies. Or maybe you still love them despite their weird compulsion to kill, dont agree with it, but are more aware/can understand/and accept the fact that they're liars, then the big conflict of the relationship was the lie (though the obvious moral conflict would be.. dude, you're killing people).
This is a complicated situation, because there can be many, many variations of it.. ignorance isn't something we can always prevent.. thats why most people probably wait for months or years before they decide on if they truly love their other.. but even then, people can be deceptive. My adivce? Do your best to decide if you love someone after you believe you truly know them (which you could be years and years and years, or months and months and months, or even days if your significant other is simple.
Q:: I feel this way about my mother. Is that gross?
A: True love isn't bound to the person you have sex with or are married to. You can love friends and family too.