Yeats classifies “passionate intensity” as worse than a “lack of conviction” because A “passionate intensity” for certain ideals can be destructive.
Yeats refers in this poem ,"The Second Coming" , to the consequences of WWI. Therefore, the ideals leading the war are destructive and do a lot of harm with passion. He says that good people - The best - are the ones who do not have any conviction. They are at a loss. They suffer the consequences of the " passionate" war ideals.