I am less well versed in the terms of what would count as mature in terms of violence than in terms of sexual content, however I do have some horror comics so I have a little to think back on...
So from what I can see he's covering it mostly or we basically just see a rather simple not-too-graphic slit and some blood. To me that looks like something that isn't really needing a Mature mark.
If your content dwelves into the following I believe you would absolutely need one:
Heavy bloodshed (the one you showed would probably count as medium amounts, whereas say a paper cut is small amounts. Heavy bloodshed would be tons of bodies lying around in basically a pool of blood, or someone graphically bleeding out entirely.)
Panels showing sharp objects entering bodyparts, cutting off bodyparts, or closeups of graphically detailed gunshot wounds
Panels showing bodyparts being ripped out (bowels, eyes, limbs being pulled off)
That kind of stuff. I am basing this sortof off of how content in my comic Crimson Tracks was mature marked by Tapas themselves and what I've seen in other horror comics on Tapas that are big enough to be reported if they mess up (or are in contract with tapas), but I could be wrong!
This being said, you are always free to mark pages as mature even if they're not REQUIRED to be so. You can do so to stay on the safe side, to protect your audience, etc. There are many reasons. I have mature content marked pages that contain things that don't require one before, when I know the characters are discussing or implying something that can get a little too much under the skin for some readers. Of course I don't do this with all, not even MOST, of the times that "offensive" topics are mentioned, I have just done it when it comes suddenly, when it isn't expected of the comic, or when there's a sudden escalation.
