Of course, the main problem I have with my OWN theory is that Satan was a liar and murderer from the beginning. My theory says that he was a morally neutral tester from the beginning, but that those tests could include lies and misinformation that could result in death. I think the verse can fit. But the verse seems to know nothing of the tester being corrupted along the way, though it doesn't negate the possibility.
Some would surely object that a test containing lies can be morally neutral or, in any degree, a 'good' thing. But I think the Scriptures teach otherwise. Let me quote a passage in Deut. 13...
1 If a prophet, or one who foretells by dreams, appears among you and announces to you a miraculous sign or wonder, 2 and if the sign or wonder of which he has spoken takes place, and he says, "Let us follow other gods" (gods you have not known) "and let us worship them," 3 you must not listen to the words of that prophet or dreamer. The LORD your God is testing you to find out whether you love him with all your heart and with all your soul. 4 It is the LORD your God you must follow, and him you must revere. Keep his commands and obey him; serve him and hold fast to him. 5 That prophet or dreamer must be put to death
We see a number of interesting things in this passage. We see a prophet who is able to do (at least seemingly) miraculous things. We see a prophet attempts to steer people toward idolatry. And who is said to be, ultimately, behind this false prophet? It says The LORD their God was 'testing' them. So is it a moral evil for God to test His people via a false prophet? Apparently not. What's more, even though God was using the false prophet as a tester, the penalty for the prophet is still death, with speaks to an earlier objection by Paidion.
Let's look at another passage from 1 Kings 22
20 And the LORD said, 'Who will entice Ahab into attacking Ramoth Gilead and going to his death there?'
"One suggested this, and another that. 21 Finally, a spirit came forward, stood before the LORD and said, 'I will entice him.'
22 " 'By what means?' the LORD asked.
" 'I will go out and be a lying spirit in the mouths of all his prophets,' he said.
" 'You will succeed in enticing him,' said the LORD. 'Go and do it.'
23 "So now the LORD has put a lying spirit in the mouths of all these prophets of yours. The LORD has decreed disaster for you."
Once again, we should note a number of interesting aspects to this story. The goal of God, here, seems to be to kill Ahab (which is certainly God's right). The means to killing Ahab ends up being the sending of a lying spirit into the mouths of prophets. So here we have God ordaining, even commanding, lies to spoken and a man to be killed. Is it too far a stretch, then, to consider the possibility that Satan could be a lying spirit and a killing spirit and, yet, still be under the ordination of God? The scripture opens the door to that possibility.