But it's far, far higher chance that terrorists would kill 10 million tomorrow than police. All it takes is for a terrorist group to get their hands on nukes.
First, getting a nuke while hard is not enough. Killing 10 million people takes either a really big nuke in just the right spot, or a large number of nukes.
Second, having a nuke is not enough you need to be able to correctly detonate on command which it which is harder than you might think.
Third, you need to transport it to a useful location as there not stored in the middle of city's.
Fourth, you need to decide using them is more useful than threating to use them.
Fifth, if your comparing them to US police then they need to target the US.
So, no it takes far more than picking one up to kill 10 million people.
PS: Simply killing lots of people is not that useful to a terrorist organization. Making nukes less useful than you might think.
China, US, India, Pakistan, Russia.....