No, it's not betrayal. He realised that he was piece of shit that leeched her off to the death. IIRC he was useless as fuck since the one who cooked was the childhood friend, the one who found foods was her, the one who found place to stay was her, the one who fought the monsters was her, etc. He was just there because she allowed him.
He being OP was later be introduced in this arc. Basically he got to learn some learn-able support skills since his role was becoming thief, enemy search, trap dismantle and trap detection. His mapping skill will help this learn-able support skills. The mapping skill is basically made him know the area around him with the radius about 1 km (or more? I forgot). With support skills that have been enhanced by this, he can detect traps, detect enemies and measure everyone's strength around that radius, and he does that simultaneously. He also can dismantle high-level magic-seal traps fast compared to average adventure. Well he is still weak af physically, so from the latest translation chapter, he is learning how to evade and attack, etc.
Despite this, he still feels like he's only helping little, well all the party members there are OP too though.