No, the legalizing of voluntary euthanasia in Belgium and Holland has not created a "slippery slope" to involuntary euthanasia as voluntary euthanasia is based upon the desire and comfort of the person to be euthanized. It is a way to allow a person a dignity in death, not a way to force death upon him/her.
Voluntary euthanasia such as in Belgium is in fact and has proven to lead to involuntary euthanasia where people who are not really in critical and serious health conditions or who suffer from less severe diseased take on the opportunity to end their life. It is leading to involuntary Euthanasia and people should not be allowed to instantly decide whether to end their life or not.
Your statement has some religious undertones and whilst I could be wrong I suspect that you are fear mongering to push your beliefs.
Involuntary euthanasia is a last resort and morality plays a very large part, it is about easing suffering not the trivial ending of lives.
Some copy pasta to consider:
The person wants to live but is killed anyway. This is usually murder but not always. Consider the following example:
A soldier has their stomach blown open by a shell burst. They are in great pain and screaming in agony. They beg the army doctor to save their life. The doctor knows that they will die in ten minutes whatever happens. As he has no painkilling drugs with him he decides to spare the soldier further pain and shoots them dead.
A person is seen at a 10th floor window of a burning building. Their clothes are on fire and fire brigade has not yet arrived. The person is screaming for help. A passer by nearby realises that within seconds the person will suffer an agonising death from burns. He has a rifle with him and shoots the screaming person dead.
A man and a woman are fleeing from a horde of alien monsters notorious for torturing human beings that they capture. They fall into a pit dug to catch them. As the monsters lower their tentacles into the pit to drag the man out he begs the woman to do something to save him. She shoots him, and then kills herself.
The morality of these and similar cases is left for the reader to think about.