BIG ASSUMPTION on my part but for the sake of diagnostics LET'S ASSUME you never ever crossed any terminals....meaning the battery cables are now and have always been connected to the correct terminal ends....as has been any charger you may have used. what you now have is either a completely shorted battery....a fried ECM....a dead short somewhere....or a huge parasitic draw from a shorted/failed component. where? simple tests as follows:
1-the battery needs to be tested properly. i'll tell you right up front that if it's some POS chinese thing...plan on replacing it.
i've seen these turds show a full charge...fool a tender...and still have either a dead cell or internal short. give it approx. 6 amps for 1/2 hour then immediately have it properly load tested.
2-your fuel pump is receiving a false ground. you need to isolate how this is happening.
3-do a full "parasitic loss" test. REMOVE EVERY SINGLE FUSE FROM FUSEBOX AND LEAVE RELAYS IN PLACE.
get a quality multi-meter and disconect the NEGATIVE BATTERY CABLE from battery ONLY. set meter to DC AMPS. place one pin of meter to negative cable end and other pin to negative battery terminal. does NOT matter which is connected to which.
you should see virtually 0 amps draw. now with key and kill switch both OFF start to replace each fuse one-by-one with the FUEL PUMP FUSE AND THE BATTERY FUSE being the last two.
4-if your meter shows good results meaning virtually 0 amperage draw then lastly replace the battery fuse before the pump fuse. ok? continue on. high draw? faulty voltage regulator. now replace the pump fuse. high draw and pump kicks on? you've found the source of the problem. can either be a pump assembly with a shorted internal wire to housing....shorted pump wiring harness...faulty ECM.
if the above confuses you get help from local electrical guru. if you feel confident then watch this guy. same procedure i've outlined above.
NOTE: some may chime in with "just remove pump fuse and test" or "just unplug pump from harness and test". well intended advice but this does NOT isolate and rule either a false ground or another short somewhere in system causing your problem. make sense????
LASTLY....when key is ON it allows the kill switch, when turned to ON...to activate the ignition relay. extremely rare but years ago ran into an instance where a "sticking/faulty" relay caused the pump to randomly activate regardless of ignition switch/kill switch position. if in doubt replace the relay.
you're welcome.
https://www.youtube.com/watch?v=XH2wdHBZ6eE