What he said.
Besides, even if Malvina-style psychopaths do show up on a semi-regulart basis, it is still very difficult to rise to leadership positions on combat skill alone. Even in a martial society like the Clans, you still need allies, backers, and patrons if you expect to get very far. I'm pretty sure officers are allowed to refuse trial challenges from warriors of lesser rank, so a true nutter can't just challenge every superior he/she gets until they reach the rank of Galaxy Commander. Warriors are often ordered to undergo Trials of Position, and if nobody will sponser you and arrange for those trials to happen, promotions come rarely, if at all. Once you do reach a Cluster command, you must engage in politics and other non-combat jockeying for position, otherwise your unit will not be sent on missions where you can gain glory for further promotion. You have to catch the Galaxy Commander's eye for him to give you combat opportunities, or for him to recommend you and your unit to the Khans for high-level assingments. No fights, no glory, no promotions. And of course, you cannot (normally) reach the level of Khan without building a support base among your Clan, since that position is achieved by election rather than Trial.
In Malvina's case, she had such a patron in the form of her far more stable brother. He was able to climb the ranks normally, and his loyalty to his sister allowed her to follow him up the ladder. When he died she took his place, and could use her brand of violence from there.