First it would seem necessary to determine if killing is justifiable and under what circumstances. If the killing is deemed justifiable, then the method may or may not matter. If the use of remote devices to kill can save many lives, then it would seem to be justifiable. If it can be done with low risk or no risk to military personnel, and doesn't carry the risk of civilian casualties it would seem justifiable. If you are talking about dirty bombs, poison gas or some other method which causes wide-spread casualties, it would not.
Killing humans using remote devices makes things safer for those doing the attacking. If there are hidden dangers such as anti-aircraft fire, then drones can be destroyed without loss of human life. Remote devices are smaller and more compact because they don't need life support or space for human drivers and pilots, which means they weigh less, need less fuel and can run quieter.
There is a justifiable reason to kill humans using remote devices. If two nations are at war with each other, either side may use unmanned drone airplanes to attack the other side to minimize the amount of casualties that would otherwise result in manned air attacks. It is justifiable to use remote devices such as drone airplanes to kill the enemy because it minimizes the chance of human casualty during the attack.
No, there is not a good reason why you should ever kill a human using a remote device. I think that if someone is killed it should be by a persons own hand, so that they will have to remember what they did for the rest of their own lives.
No, there is not a justifiable reason to kill humans with remote devices since there is not a justifiable reason to kill humans. All humans are born with the inalienable right to life. This right must be respected. To kill a person is to rob them of this right, and that is one of the worst abominations anyone can commit.