The reason Ethan stalked Felix was to see what the Ancient Evil Spirit inside him was planning to do.
However, when he realised that the Ancient Evil Spirit wanted to vent its anger on the innocent mortals for the missing Night Demons in the towns, Ethan had no choice but to act.
The reason for the missing Night Demons in the towns was obviously Ethan, and moreover, since Ethan had the power to stop the Ancient Evil Spirit, he decided to act.
'I have to kill the Ancient Evil Spirit before it can even realise it.'
Even though Ethan was confident in taking down the Ancient Evil Spirit inside Felix, he didn't want to give that spirit a chance to act.
Ancient Evil Spirits had mysterious methods and techniques, and Ethan believed it was better to kill it before it could even use them.
