Despite me wishing for Lucas to kill the assassin too since he'll be troublesome later on I can give him a break based on three observations:
- First, he spent his entire life living as an low rank adventurer doing menial tasks like collecting herbs, killing small fries or being a mule for some random party while getting laughed at, so I really doubt that he ever met an assassin before and much less knew how to deal with them
- Second, he was superior to the assassin in every matter even after the assassin buffed himself to become stronger and faster, this coupled with his ignorance of dealing with assassins made him lower his guard
- Third, he wanted to interrogate the man to gather more info and try to clear up his wife's house, even now it's their objective
On another hand Aria is becoming more and more cute and sexy, damn, that look she gave Lucas was a killer one, I bet she's secretly a pervert and now that she awakened herself to the pleasures of sex she'll become even more pervert when Lucas gets more wives.