I've personally never seen it as "playing filthy." You see, both teams are playing to win. This will include roaming, ganking, and helping their teammates get leads when possible. Knowing that your opponent will use these tactics against you, you must then adapt to use these same tactics against them in order to keep things fair.
Being a jungle main, I have several times been yelled at for making the game "unfair" by ganking my teammates. But that's part of my job as the jungler: giving my teammates an advantage. I try to give kills when possible, if I don't risk them killing my teammate or escaping by doing so (I had one time where I went to give a kill from a gank on Nunu and my Aftershock procced the damage portion of it, killing the target on accident. Funny moments like these are good). There will be games where I get outjungled and my opponent takes all the objectives and ganks every lane enough to get their team ahead. There will also be times when I do the same.
Let's use Fiora as an example for something for you to think about. Fiora, generally, has a pretty weak early game, particularly when dealing with other bruisers. However, she turns any matchup that she is in into a skill matchup based around her W: Riposte. Riposte makes Fiora immune to everything for 0.75 seconds before stabbing her blade out in a line in front of her, damaging and slowing enemies for 1 second. If she blocked a hard crowd control effect (basically anything that prevents her from moving, like a stun, knockup, or snare) in the duration, she instead stuns the first champion hit.
The existence of a solid amount of % max health true damage and healing in her passive makes her a strong pick into tanks, despite her weakness to bruisers. Not to mention, tanks tend to have more hard crowd control effects, which is normally an asset when dealing with bruisers. A good Fiora, on the other hand, can stomp tanks consistently due to her % max health true damage, healing, and stun.
But the big thing about Fiora is her split push and 1v1 potential. A Fiora playing her best will never be killed in a 1v1 in the late game, and she will split push and kill your base before you're even able to react. It always takes more than 1 person to stop a late game Fiora, and her healing and ultimate give her the ability to rack up big multikills if the enemy team doesn't send enough people her way. Because of this late game, it may draw junglers to come gank a Fiora while she's in her weak lane phase. On the enemy side, they will try to shut her down. On the ally side, they will try to get her a lead and get her to that unstoppable late game faster. And trust me, you do NOT want to see a Fiora on 2 items at 15 minutes.
While her early game matchup is entirely a skill-based matchup, the later the game goes, the more she'll be able to kill any single enemy that comes to try to fight her, and the more single enemies she'll be able to kill alone before she goes down if multiple come to fight her, and pretty much all of these at this point depend on the Fiora's skill alone.
