KILLER BRISTLENOSE..!!!

We moved over 3 months ago, unfortunately I had to shut down my other tank.
Had in big tank - 8 Guppies, 5 Glass Cats, 5 Cardinal Tetras, 1 Female BN & 1 Male BN, Heavily planted with 3 caves.

My young male BN was put into big tank with the others and found dead in the older BN's cave entrance about two weeks later.

I found two young longfin BN's (sex unknown) at good price and HAD to have them! Should have waited until other tank was set up again but didn't. About the two weeks later again, one of the longfins was found in older BN's cave entrance again, the other one is fine (am guessing now it's a female).

THEN my brother moved overseas, I inherited his 2 Borneo Suckers three weeks ago. I have just found the remains of one at HIS cave entrance again.!! (guess the remaining one is also a female)

I cannot see that any toher fish are the culprits and can anyone tell me why is this male killing off the others?

Male BN will defend their territory to the death of one or the other, this is not unusual at all. I wouldnt be adding any more males to this tank.

Wow, that is insane! I currently house 3 males together. I can definitely see the dominance hierarchy but no fighting to the death.

I haven't knowingly added males, I can't sex young ones but HE ovbiously knows    
Is there any tricks to tell them apart when they are young?

Thanks Chaase, everything else I've read leads to "it must be another reason" or "they are scavengers NOT killers" but as it's now happened 3 times, I just cannot believe that! Coincidence only stretches SO far.

I obviously cant wait to set up my other tanks again, I'm sick of losing fish and build is taking longer than expected..

Can you tell how the water circulate in the tank? Is it possible that current brought the dead fish by the cave?
My plecs always pick their spots so the water brings the food to them.
Did you test your water?
I find hard to believe that young ones (so young that you couldn't tell the gender) would pick a fight with adult male on his territory.

Thanks Kaban. Am running an Ehiem Professional 3 lengthwise across the top, his cave is front middle, with entrance facing the outlet. So the water is pushed across the top, hits the opp side, and sweeps down behind his cave. So no I dont believe it would be the current.
Water parameters all fine & no problems with any other fish.
I dont think they are picking fights, just getting bit too close to his territory and ambushed perhaps  

I just had a shortfin green dragon bristlenose female kill a longfin female because she wanted to spawn.