I don't think she's lying but I also don't think he's just a big asshole. He could be lashing out at the fact that he didn't want to actually do this.
Sounds more likely.
Arca has been living his life under the pitch black shadow of his parents. It really kills a lot of motivation when you grow up seeing an a huge wall you got no hope to overcome.
Being Hero could be his chance to finally move away and find purpose in his life. But he's been living sheltered, so he never know how to handle that power rush from finally being stronger than anyone else. Or, realized he can't act up to everyone's expectation of him becoming a saviour.
People in town also seems to try tolerating him, saying he did work hard to protect them from the monsters.