Would Valentino be willing to take the blame for killing Leo on Aizen's behalf? The answer, of course, is yes.
Anyone who fears death would make the wisest decision in such a situation as quickly as possible. Those who are afraid of dying are willing to gamble on anything—except for their own life.
If Valentino wasn't afraid of dying, then Aizen would truly be at a loss. Even if he were to kill Valentino, it would be pointless; the high-ranking officials of the Left Clan would ultimately hold Aizen responsible for Leo's death.
Of course, at that time, Aizen could also prove his strength and his value. With both Leo and Valentino dead, the Left Clan, in need of someone to take charge of the Black-Clothed Disciples' area, might temporarily spare Aizen and allow him to atone for his sins by continuing to compete for resources for the sect.
