Ruins:

Sans dosnt kill any monsters in the Ruins he only helps them attack and when there about to die sans flees the battle but on the final kill before landing the kill shot sans kills the last monster right in front of frisk.

when toriel takes frisk in the home sans dose hide near the ruins exit and then he makes toriel go there and kills her and when frisk goes near the exit he sees Sans pealing her skin of its skull and wearing it and then the fight begins.

Sans uses some of toriels attack due to in my take if sans has killed a boss monster that has a special attack type he can use it, also sans pertends to act like toriel guilt tripping Frisk and when the battle ends he flees leaving her face and clothing there, and where in normal undertale after the toriel fight you would see flowey, Sans is there and he tells you why he's doing this and then he slams Frisk face into the ground causing a concussion and then when there on the ground in pain sans just leaves flowey then pops out of the ground and he knows what Frisk has done but flowey offers Frisk a chance to make things right and Frisk agrees as flowey banaids them up as they exit the ruins.

Snowdin:

so up until after the papyrus fight you don't see sans once in snowdin so its just you hanging out with papyrus and unlike regular geno you do some of papyrus puzzles not all of them but most of them also Frisk only kills about 10 people

including dogi and doggo and snowdrake, but after the papyrus encounter starts and in the battle you do see sans in the background a bit, but then after the battle Frisk Vomits blood because of the nausea because of the fight and that frisk still is shaken up after what happened in the ruins, papyrus heal up frisk and hugs them as frisk cry's but papyrus ask who did this to them flowey tells him that it was Sans who did this and when papyrus confronts sans about it he tries to kill Frisk but papyrus hold him off as Frisk and flowey hide in papyrus garage and they fall asleep there and Frisk has a nightmare where sans dose kill them in many brutal wars but they wake up and when they check to see where papyrus is all that's left is his scarf so frisk puts it on and just continues into waterfall.

Waterfall:

So In Waterfall Frisk is still LV 6 and stops killing as they don't feel like it so they just go though waterfall and Undyne doesn't show up at all as sans already killed her luckly monster kid is there with frisk in waterfall but when they encounter sans he throws monster kid off the side of the bridge (don't worry he survived) but after that is where the next sans encounter starts and sans dose use undyne attacks and toriel attacks and when the fight ends sans tries to kill frisk with a blaster shot but Flowey takes the blow and at then Frisk just breaks down a then murmurs to sans "please kill me" which is when sans realises that he's gone too far so he stops trying to kill frisk and tries to stay away from them but he will keep killing because he knows he's to far gone to stop

Hotland:

In Hotland Frisk dose meet up with Alphys as she dose feel bad for frisk so She helps frisk though out hotland but they do kill a few small monsters to get up to level 8 and stop there but then in the core they see sans killing Mettaton Neo but sans dosnt fight them as before he leaves he just says "im sorry" Alphys dose watch and says that she has done all that she can but now it is up to Frisk to stop sans, and they meet in the corridor and then the rest is history (aka will be in the fangame)

Bonus Stuff:

Sans Misgenders Frisk a lot when talking to them.

Frisk in In Snowdin to Hotland talks shy a quietly to everyone

Also In Vania to Necroptosis Sans Doesn't want to fight anymore but the reason why he dose is because he's thinks he's too far gone to change

In Necroptosis Sans dose use the 6 Souls which explains the background colors

Also Sans is the only one there as Asgore, Flowey, Toriel, Papyrus and Undyne aren't real as they are just hallucinations

Frisk Slashed Sans in Self Defence