> But negligence is fundamentally different from mens rea

It differs in that mens rea is the legal concept of a culpable state of mind, and negligence is one example. More fully, a crime is generally defined by a prohibited act (actua reus) and a wrongful state of mind (mens rea), though there are strict liability crimes with no mens rea required.

For, say, murder (in common law, specific statutory schemes may diverge from this somewhat), the actus reus is homicide, and the mens rea is “malice aforethought”.

While “malice aforethought” is sui generis and seen only in murder, the common kinds of mens rea used in defining crimes, in descending order of the severity with which they are usually treated, are intent, recklessness, and negligence. (The same mental states are relevant in tort liability, though strict liability in tort is more common, and the civil and criminal definitions of negligence, particularly, are somewhat different.)