Aggressive Molly

Hi!
I have a male yellow molly that has been biting the tails of the other Mollies in the tank. The first time I noticed it, I placed him in our other tank where he remained at the bottom hiding. I felt bad and put him back in the big tank and he started swimming around again, the next day he killed a molly…

Ive put him back in the second tetra tank for a few days now and he just remains laying at the bottom.. what do I do!!
The tail of one of my mollies started growing back while the yellow was away in quarantine

Get rid of him because he won't change. Some fish are naturally more aggressive than others and if you have one that is harming others, get rid of it.

They look like guppies to me

Those are guppies, not mollies.

Males can vary in their level of harrassment to other males, some not bothering at all, some super aggressive and dominant, but not to the point of "killing" another one in a direct attack kind of way, more that they're relentless in chasing their target and the target winds up dying from the stress and exhaustion. Keep that yellow male guppy away from the others for now, but I'd either return him, or find him a home with someone who has females and wants to breed him. Male only livebearer tanks CAN work, but sometimes there is an occasional male who is just too driven and dominant to handle it, and disrupts the whole tank. Sounds like he might be one of those.

Can you please list and show all the fish in that tank please, and a photo of the tank as a whole? Are the rest all males, or females too? There are some tips and tricks to reduce problems in a male only tank.

Guppy.