If they crash due to a manufacturing defect, the manufacturer is liable; If they crash due to operator negligence, the operator is liable; if both factors are involved, both may be (potentially fully, in each case) liable, and in various circumstances the vehicle owner and/or driver's employer may also be liable.

At least, that's how it is with non-automated vehicles, but I see no reason the same rules don't work fine for automated vehicles.

> Can the developer be sued for negligent homicide/vehicular manslaughter because the software had a bug?

Generally, vehicular manslaughter is a special offense that applies only to vehicle operators. Anything from negligent homicide up through depraved heart murder could, in principle, be applicable to software (or other) defect, depending on knowledge and other factors.