Stephen had access to a night kill.
He either got it from
a) Being a vampire
or
b) being a member of a faction
If he got it from being a member of a faction, the factions he is likely a part of are:
1) Watch
2) Undead
3) Foreigners/Ubervaldians
1 is extremely unlikely, since it runs counter to the Watch's victory condition (and you claim you know nothing of a Watch NK).
2 is possible, even probable.
3 is a slim possibility, but we have no evidence of a foreigner faction.
So the most likely reasons Stephen had a Night Kill are either:
he was a vampire
he was a member of the undead faction
Both of which apply to Yunalesca. Therefore, Yuna likely has a Night Kill, therefore, she is a good candidate to lynch.