2004 cadillac deville wont start or crank Help plz

My cadillac won't start. When i turn the key the engine wont turn over and
when i do
turn the key all the way the dashboard lights all turn off. Also on the gear
shifter there
is no longer a line under the gear showing what gear it was in. If i move the
gear shift
up and down a few times hard-ish the line comes back so i put it back in to
park and
tried again and still nothing and then the line under the P disappears again.
The line
doesn't always come back after messing with the shifter. At first i thought
maybe my
battery was dead so i tried to jump it and after that when i tried to start it it
still would
not start. This time when i tried to start my car, it started beeping at me (the
same
beating that happens when you leave the key in) and locks the doors on
itself, then, on
the dash board screen it says "service theft system" then the over head
lights started
to flicker and the light on my dashboard for my brights lite up and started
flashing and
then stopped maybe 30 secs later. but slowly the battery died the rest of
the way to
the point that the dash didn't even light up at all. So i took the battery out
from under
the backseat and put it on a charger to charge. once it said it was charged i
put it back
in to see if it helped, it still wouldn't start. It is possible that the battery could
be bad
but i want to make sure its not something else before buying a new battery.
It sounds
likes its a mixture of a bunch of things and I'm not sure what to do. Help
please.