I admit, I've been keeping secret thoughts from someone I care about for a long time now.
If you care about them, why keep your thoughts secret from them?
Because they aren't nice thoughts; I want to be mean to them.
It isn't necessary that you tell every single one of your thoughts to your loved ones. It is normal to have unkind thoughts sometimes, and it is wisest to keep those kinds of thoughts to yourself.
I know. I'm sure this person isn't going to understand my thoughts.
They might understand them, or they might not. The important thing is that they would be hurt either way when it's not necessary to hurt them. When sharing thoughts would serve no purpose and just be cruel, it's better to be kind and keep them to yourself.