There is only one way to make data unrecoverable. Even writing zeroes, computer forensics would be able to recover data due to magnetic residual traces. The only way is to write over it. That is what killdisk does. It writes a patter of 1's and zeroes in multiple sweeps. That way when zeroing out a drive, if a zero was once a 1 it will still have residual trace. But if killdisk comes bye and writes patterns, say like this:
0000 0000 1111 1111 0000 1111 1111 0000 0000 0000 1111 1111
etc and then the next sweep reverse it, then if they ever do recover the data it will be completely scrambled mess and unrecognizable as nothing but crap. After that, you can write zeroes and they can try to pick it up all they want from forensics, it'll be nothing but a bunch of garbage.... that is until they get more sophisticated and touchy equipment...