Oni_punch_man
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First I have to say NO ninjas feel guilt, the only ones that would feel guilt are the ones that couldn't save their families or friends (obito, the one eyed masked pervert). They were trained for this with subtle mind control. Second, All of them are killers even a certain ball of sunshine and hinata. The only mention of anything near is hinata saying she dislikes fighting (not killing). And tsunade had a dislike or phobia of blood, so instead of guilt it would be queasiness if anything.
Even if they were to join waver can't be converted anyway as said early on in the story only 1 of each servant type was usable and caster is already used by the kuro neko. Plus shouldn't he already know about the conversions? Iris is a servant as well.