If she likes the party back, and they take her with regardless of her actions, she could kill people she perceives as threatening them - which, her being ten, may not be actual threats - the rude postmaster whose approval they need to dock, the man flirting with a party member over drinks, the fighter who bested one of the party in a friendly sparring match, etc. Basically continue on her established pattern of offing people with only a 50-50 chance of it being beneficial, the other half the time it causes problems for the party.

If they don’t take any steps to curb this behavior or rehabilitate her, have her advance to trying to kill party members the next time there’s a major intraparty disagreement or someone does something she doesn’t like.