As said already it is wrong.
When playing a Single Player game the only person you are affecting primarily by using a trainer or cheats is yourself.
When you do it online, you are affecting everyone you come into contact with. You may "enjoy" yourself but the people you play against aren't.
It's like using an aimbot and espbot in FPS MP matches, you have a huge advantage over others, so people are getting head shotted constantly, you think they are happy about it, not only is it easy to spot people using scripts/bots/hacks it infuriates them, and in turn may not pay for the next product from the developer/publisher. So people like yourself who do use hacks are ruining the game.
You may think to yourself you aren't ruining anything, you are very wrong. Lets see how long it would take you to give up on a game if most people you came up against is using hacks, not very long you would be stamping your feet about it. My guess you have experienced this already hence why you came here without reading the rules and asked for a hack to an online game.
Not only that, it proves you are crap at the game also.