Hm, I actually donīt see the problem. Youma are created by being infected from the parasite, which grants them super-human power. To fight Youma, Claymores insert part of a Youma inside themselves to gain access to these powers. However, the Youma-flesh doesnīt lose its infected effect, so over time, the Claymores go nuts. In which case theyīre asking another warrior to kill them.
Of course, the whole procedure is weird.