About the whole "I don't understand why they are so mad at Finn who was nobody special and just a grunt."
Well. That would be slightly true, to an extent, but Finn interfered with the First Orders shit. If Finn just disappeared and didn't aid BB-8, Poe, or Rey. The First Order would still want the scumbag to be killed, but he wouldn't be top priority.
But Finn interfered.
First. This grunt saves the best Pilot in the Resistance, helps destroy equipment of the First Order (when they are leaving in a Tie Fighter.) So now he has done that, Snoke finds out and Kylo and General Hux aren't pleased. Especially Hux with his whole "I have trained them since they were children thing". So now the General is pissed, and who is on the short end of that angry leash? The CO's, and now the CO's are pissed because the General chewed them out. So who is on their leash, the NCO's, and now the Soldiers. So now these Soldiers are probably chewed out, considered worthless and ineffective because of one damn guy, so now any soldier that sees Finn wants to kick Finns ass for all the pain and suffering they are going to have to endure. (This wasn't shown, but you know. Just kind of figures because of the First Order and Military shit.)
So now they hate Finn.
Now Finn saves BB-8 from the First Order and escapes. Rey doesn't matter at this point because they don't know she can use the Force, so she isn't that big of a deal at the moment.
So basically. The more shit Finn added on, just pissed the First Order's chain of command off even more until it fizzled down to the regular ol' Cannon Fodder Troopers.
This is probably why TR-8R (as they call him) threw down his weapons instead of just shooting Finn in the back like every other scum on that planet. He had a grudge on Finn because everything has done, has pissed off that Soldiers superiors, and he is getting his ass reamed because of Finn.
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Of course, this is all just speculation, and my personal view on what was going on in the parts they didn't show. I could be wrong.