6:05 PM PT -- 50 just walked his words back a bit, saying, "I have no ill will towards anyone living on this Earth. The people Shaniquois son looks up to and associates with is a reflection of the negative energy he embraces towards me. I paid his mother $1,360,000 in support to afford him opportunities I never had. They just spent it, over the years I repeated get a job now they have to figure it out."

50 Cent's clearly not going for any "Father of the Year" awards ... by making it known he'd be just fine if his oldest son got injured, possibly even killed, by a bus.

Fif left the shockingly cruel comment on an Instagram pic of his 21-year-old son Marquise Jackson hanging out with the son of Supreme McGriff -- one of 50's rivals.

The rapper wrote ... "if both these little n****s got hit by a bus, I wouldn't have a bad day."

Damn.

The bad blood between father and son goes back several years, stemming from the bitter relationship between 50 and Marquise's mother, Shaniqua Tompkins, who sued the rapper for $50 mil. The suit was tossed in 2009.

Since then, Fiddy and Marquise have taken shots at each other publicly on social media and in their music ... and Marquise told us last year his dad's music sucks.

As for the hate for the other guy in the photo ... it presumably comes from 50 Cent's longstanding beef with his father, Kenneth "Supreme" McGriff. The 2 are archenemies, and Griff -- a former Ja Rule and Irv Gotti associate -- allegedly once plotted to kill 50. Supreme is currently serving a life sentence for murder conspiracy and drug trafficking.