Heat, Tension and Passion
Ashton wants to thin out the herd of demons above, he truly does. The only reason he hasn't done it yet is due to the fact that it'd be extremely risky.
Every single day, Ashton would allocate some time to venture above and observe the demons roaming around the ruins of City M. There's way too many of them for him to comfortably walk around even at daytime, which proves rather risky to even think about taking the initiative to attack them.
Ashton had no doubts that the moment he fires a single shot, he will be mobbed by hoards of demons. And even though he has the ability to kill the most common types of Demons by now, he is nowhere near strong enough to deal with a massive hoard on his own.
Sadly, that's what kept him rooted underground. What else can he do other than to bide his time and keep training until he's strong enough to deal with all of the bullshit above him?