1996 Mazda B2300 battery

My light was left on in my truck over night an the battery died of course. Its been on a charger for over 12 hours now and it still isn't starting? My battery has died numerous times an it has always started after I charge it! I have an aftermarket alarm system but I have cut the wires so it shouldn't work, but the light on my dash is constantly blinking as well as a noise that sounds like my blinker. Is it just my battery? Do I need a new one? Does the alarm system have anything to do with the problem?