"I look out for myself, Elena. I'll always look out for myself. If you're smart, you'll do the same..."
Honestly, I think we can tell alot about Katherine from this line but, unlike what seems to be majority opinion, I don't think that this quote is as important to Katherine's character as many others. That said, what she says makes sense.
When Katherine trusted Elijah and found out that the ritual would still go on, she made a run for it. She couldn't trust much of anyone after that. Even poor Trevor, who - I imagine - wouldn't have betrayed her trust at all couldn't be trusted. Or, at least, that's the way I can see Katherine looking at the situation.
Nowaday it's very much the same. The Salvatores are unique individuals (or Elena's just really, really special ) to be willing to put their lives at risk in order to save a young girl from harm, and Katherine - in 1864 - had been running so long that she couldn't count on a couple of new Wanyonya damu to protect her, much less after about four hundred years of running; wewe have to wonder how close Klaus au Elijah may have gotten in the meantime.
Katherine has had to take care of herself. Not looking out for someone else (not "letting upendo get in the way") and not trusting anyone else to look out for her has led her to be very self-sufficient, but also very survival oriented.
It's almost the way you'd look at a feral dog; yes, a feral dog doesn't have social skills au that natural affinity for human affection, but that feral dog could surviva much longer in the wilderness than a domesticated dog could. It's sad, and kubadilishwa, unaweza kurudiwa tena if someone knows how to turn it around, but it makes sense in the name of one's survival.