From all I can ascertain, Alexander the coppersmith (and it simply isn't known if this the same Alexander mentioned in 1st Timothy 1:20) did willful, deliberate, malevolent harm to the Apostle, whereas those who (perhaps) could have stood beside Paul at his own trial did not. Negligent, but passively so, acting out of fear or ignorance perhaps; but not malevolence, not hostility. Paul's request for mercy towards them is reminiscent of Christ's prayer on the cross for the Roman soldiers (but not for the malevolent Jews upon whom He laid the guilt of all innocent blood from Abel to Zechariah - see Matthew 23:35), just as the Apostle's prayer for Alexander is reminiscent of the Lord's judgment against His betrayers in Matthew 23 and 24.

Paul was following the Lord's example in both verses (2nd Timothy 4:14 and 16.

-R