ILS into SLLP - Landed in the field before the airport?

I was flying the default King Air 350 into SLLP. I intercepted what I thought was the localizer, The autopilot flew me around a little to get on the right heading. The plane starts descending, me figuring I hit the glidescope and was being taken in for a landing (the glidescope indicator was working). Well it turns out that I was never going to make the runway and I tried to land short in the field prior to the runway (probably about 1 mile or so). I ended up crashing (it was too late when I realized what was going on). I reloaded my approach and decided to manually fly it. This time I held it on the correct heading and had the runway in sight. I hit the glidescope at a different point and it guided me in for a good landing.Can anyone help me with why the autopilot would land me short of the runway (I know that it won't autoland, but I usually let the autopilot take me in most of the way), when I can manually fly and everything appears to be working fine? I know this will probably be summed up to pilot error, but I could use some help on this (me and ILS landings aren't getting along very well). Thanks for you help.