Here this tells you how to report [dhs.gov]

You can simply call 911 and speak to your local PD. I count this is as urgent. I didn't realize the fella was lurking the parking lot.

He apparently has motive - we don't know if he has weapons - but we know he had a plan. So he must be reported to authorities.

I worked in Mental Health and we'd have put him in a ward to get some much needed treatment at this point. (Hopefully they'd also confiscate whatever weapons he has as well).

Sounds like an instance ripe for a red flag law.

I don't think you said he's actually on the hit list?

But I would say no matter what the store has done unless he's locked up or under secure treatment somewhere there is zero to stop him showing up guns-a-blazing anyway. Maybe the actions he took were of desperation and he wanted to be locked up for his own good?

I wouldn't go back to shop there or work there until the authorities had done more with that employee. It's crazy to let him slide like that, risking everybody's lives. The public should be kept updated by the authorities about how he is being managed. Not left to guess whether or not they are safe.

Ask to speak to a manager - there may have been more done than you think. The person may be in treatment. It's highly unlikely that they'd have done all that and be left to their own devices.

Simply state - "I'm concerned about shopping at my area Walmart after seeing this post to FB - can you tell me what actions were taken?". I'm rather surprised they didn't counter post - but they may be afraid of a lawsuit.

When you say his mom is terminally ill and "he's been watching the area" do you mean he moved to the area? I'm confused by that statement.

If you believe this is an ongoing problem that hasn't been remedied you can phone your local PD. They won't mind. They'd rather have concerned citizen calls than never hear about it.