He's a demon lord in his past life, right? He's in the wrong path, doing bad deeds and thus, chased by the heroes. In the novel, before he died, the hero told him that if ever he had another life, he should try doing good deeds instead. Now, when he returned to the past, he finds it boring to do all the things he did in his previous life like being a demon lord, he wants to experience new things and he remembered what the hero said to him about doing good deeds and that became his motivation. It's also his way of recognizing the hero's strength/showing respect. 

I recommend that you read the novel to better understand Zich and the story 'cause they cut some of the parts. This story is a good read and there's a twist that you never expected.