Hostiles heading towards you are more threatening than ones heading away from you. To figure this out, take the dot product of the velocity vector with the normliased position difference.
( Me.pos - Them.pos ).Normalised() * Them.vel
the larger the positive value the more threatening it is. If it's negative, then its not really a threat at all.
unless... it's changing direction....
find out if it's intention is you by taking it's acceleration...
( Me.pos - Them.pos ).Normalised() * Them.acc
If that's positive, you should probably start running. But if it's negative, then maybe it's frightened of you and you can stand your ground, or even advance, because it's trying to back off.