By the very nature of the case, you can't impose non-existence on anybody. There has to be an object of the imposition, and no object exists. However, discarding or harvesting cells from embryos is most definitely imposing death upon them, and that is wrong. A "position that, while bringing life to some, also imposes death on others" is a classic example of "playing God", which is far worse than my supposed "pontifications".
Ah, but insofar as there is a potential for human life, there is an object upon which you can impose something. For example, of one partakes of a drug that is known to create birth defects, even before reproducing, and only afterwards reproduced and the result of this reproduction is a child with birth defects, that person can be said to have imposed them on their child, even though when the actions were performed (both the action of reproduction and the action of creating a high likelyhood of birth defects) prior to the child's existance. You can impose on something that potentially may be as much as you can impose on something that is.
Of course, making decisions that result in life or death for humans is not playing God, men have done that throughout the ages, it may be a decision that results from one having power, but it is a human power, not a divine one. However, to pontificate on issues of existance and non-existance, that is going yet a step further, that truly is an attempt at playing God.