I'm going to use tinder despite already being in a relationship with someone.
Why would you use tinder then?
It is something fun to do to take up time.
You are giving people the wrong impression if you are on there while being in a relationship. How would your girlfriend feel?
What? My gf has never felt neglected because I'm always on the computer. Spending some more time on Tinder won't kill her, she might even be fine with it.
I don't think she would be fine with it. In her eyes, it is like you are looking for a new partner already. Find something else to do with your time.