While planning to take Revenge on someone is probably not the most Rational of things to do, Clare wanting to do so is something I totally can understand. I talked about this before, but Clare's world is a bit different from our own; there is no clear way to get Justice in it. The Organization, only kills Yoma for money. There are no courts or law enforcement that are clearly seen in the story, and besides even if there was there would be nothing they could do about Priscilla. Clare had taken away the one person that was her everything in the world, so she throws out rationality in favor for loyalty. I can quite honestly say that I would do the same, if we didn't have a justice system in place that takes care of these things. Irvine is right; Clare said it herself, that she couldn't keep Teresa, that she wasn't strong enough to keep on living as a human once she lost her. Clare is not perfect, and I don't like perfect characters, because it is character flaws that make characters interesting. Clare had lost her everything and being loyal to Teresa wanted to avenge her, so she gave up everything to avenge the one person that mattered most to her, and that was her whole purpose of living until Raki gave her another. But anyway, Clare is not irrational while trying to seek her revenge in that she makes sure that other people don't get negatively impacted by her goal. She puts her quest for revenge on hold to save fellow Claymores. She has a higher priority in finding Raki than seeking her revenge. The concept itself is irrational, but Clare generally makes sure she doesn't endanger others by her actions, IMO.