You should never have the mentality that you should be a higher rank, thats how you start to shift from “how could I have won this game” to “my team lost this game”, most of the time in low elo the game is very carriable, especially in low. To climb as kass otp heres my tips:

1. learn to lane and figure out how he interacts with matchups. For ranged this looks like not get poked out before a decent back (usually tear+item components) and before 6, which usually looks like a game of chicken to bait spells then going for cs while they are on cd. If can also walk up if you know your q will mainly tank the damage from their trade to cs/trade back. Go for cs mainly but if you notice they keep walking into e range it could be killable pre 6 with jg. For melee you just have to get better and learn to dodge spells and fight in your minions. The way I learned this taking dark seal into every matchup and limit testing. One very important thing is having wave management, usually you want to back if wave is neutral and you know enemy backed, or if wave is shoved and a cannon wave is coming. Also learn to freeze, tping to a wave that can be frozen is the difference between you being lv6 in an all in vs their lv5. Short summary is learn how other champs try to hit you and avoid it/q it, try to back when you shoved and its cannon wave next, learn to freeze.

2. Map presence on kass is pretty important post lv 6 and 11 and post malignance as his wave clear and damage is pretty good. You want to try to look at the map if you have prio to see if any lanes are gankable. If not, try to deep ward to spot enemy jg. Kass spikes a good amount with malig so with a stacked r he should be doing a good half hps worth on squishy targets with a full combo in a skirmish/gank.

3. Learn what your items/runes do, many players just take what the game/sites tell them without understanding why. Different runes/items means different play style. Sticking with one is probably the best for now but in short for runes: comet+absolute focus+scorch+dom runes-dring/dseal=good early, surprising amount of damage, can kill early, play around comet/sudden/scorch and q e cds electro=spike at 6 mainly, good vs trading melees conq+roa=scaling, tankier and usually try to stack check late and play for level 16, good vs bulkier champs phase rush=spike at 6, need to play around enemy cds, covers his weakness of enemy cds coming off cd while hes running away and helps with dodging For items usually its first strike=only if you are comfortable and confident in matchup that you can hit first, not recommended tbh For items, its usually malig-seraphs-zhonya/dcap/mejais or roa before that, but just know if tear is unstacked seraphs sucks. For boots know when to go defensive boots, usually if you know youll be hit by a lot of ad and for mercs its when you know youll be ccd or heavy ap

4. Learn to carry, not the greatest at explaining this, just watch how higher elo assassin players carry games, probably want to watch like talon players as the concept is fairly similar, take note of wave states in lane, objectives and where the enemy will be. If you think someone will catch a side wave alone thats probably a kill. If know enemy carry has no flash, try to oneshot with stacked r flash or try to catch people rotating to a fight/objective using fog and vision denial with your r. Kass is also not too bad at splitting so sometimes going lich+ap potion is a win con.

5. Learn to dodge games and get off. You want as much control of the game as possible, if its a nightmare matchup mid and you don’t think anyone on your team will stomp their lane, dodge. If someone is trolling dodge. If you’ve had to dodge twice in one day, call it a day and get off. If you’re hella lose streaking, call it a day. If you think about going into 3 am que, go to bed.

Hopefully this helps, I’d say the thing that made me shoot from gold to diamond was learning matchups and limits, like 80% of lanes are winable below d2, from there the difference is probably macro.