He may not have known how easy it would be for him. He probably believed that there was a babysitter that night, per his conversatin with brenda. I'm sure he didn't expect the babysitter to know the alarm codes...that might have been sheer luck on his part. (btw It is possible that bvd's conversations in their entirety with DW isn't known because mudd could have limited the release of htat info)
WRT: the computer....maybe he thought he wouldn't get caught...wouldn't surprise me. There is speculation that his son neal collected the illegal child porn and stored it in his dad's home office...automatically assuming that neal's the killer and not DW.
This bears repeating:
**FREE NINJA DAVE**FREE NINJA DAVE**FREE NINJA DAVE**