Years ago, I saw on the telly that your own saliva will remove your own bloodstains. I did try it not long after that when I cut myself shaving and got it on my white collar just before I was going out. Believe it or not, it did actually work although it wasn't a massive stain and I did have to sponge the last little bit of the stain to dry it.
The best way to get blood out of anything is to soak it in cold watr (never use warm or hot water as it sets the stain). Then wash it with a little soap and more cold water. You can also use Biotex, which is quite good for stubborn blood stains.
Vanish Oxyaction will remove it in no time at all. Either mix some up and soak the trousers in it for a short time or wet the fabric with the solution and wash normally in the machine. It's quick and painless.
Burn it...the forensics will be able to find traces of the dead persons blood no matter what you use....make sure you scrub under your finger nails as well!
Soak the garment in cold salt water overnight and then wash as normal in the washing machine. I often come home from work with bloodstains (other peoples so the saliva answer is not an option) on my shirt and this works evereytime!