If you don't care about a relationship, the other person still might, and them being hurt should matter to you.

End the relationship since you don't care about it anyways, to let the other person move on with the least lies and sense of betrayal, and then fuck everyone you wish.

Cheating isn't a way to end a relationship, cheating is lying in order to keep it longer.

That's rather selfish. There is harm, but not to you. You're okay with hurting other people for your own gain to avoid having one difficult conversation. I can only assume that you wouldn't feel good if a partner treated you like that, so why do so to them? Either you have a general lack of empathy, lack introspective ability, or are just perfectly okay with the idea of being cheated on, and also the idea of someone else hurting because of your own actions. I'm fascinated, and also recommend you try consensual polyamory next time instead.

to me there is no harm in it.

Well yeah, but you could say the same about hitting a pedestrian with your car. To you, there's no harm. But the person you're cheating on, the person who's trust you've violated, who's time you've wasted, who feels stupid and used and humiliated after the relationship is over, there's a lot of harm for them.