God's mercy doesn't supersede His righteousness, it just reveals His sovereignty. In the examples you have given, they simply show an ends justifying the means, but this too can be distorted. When justice is served to the wicked, is morality then questioned? A murderer slain, served. A liar confounded by the truth, served. Not all killing is considered sin, such as self defense, or preservation of life, but this isn't an excuse to be a serial killer. Exceptions to the rule are just that, exceptions.
The give and take of many of the moral principles we attempt to understand is the intent of the heart, and one's character. To tell a lie for selfish gain is not the same as telling a lie to save a life. Deception, for example, holds with it an evil intent but in war it is simply a distraction, a tool. Would you consider a feigned punch (a simulated or fake, punch) to be sin since you are inherently being deceptive or making your opponent believe a lie only to hit them with a real and deliberate punch?
You see, the intent of the heart is of supreme importance. Of course ignorance isn't bliss, murder is still murder, but to take the life of another to save someone, this then goes into another moral concept. A hero who defeats the bad guy is seen in a positive light, but what then is the line that determines that the retraction of life is sin? Or telling a lie? Being angry? At what point do any of these acts or emotions become sin?
While intent and motive do not necessarily juxtapose actual actions in terms of moral judgement, it seems that they are considered. God is the judge and knows all things. The Bible is, in fact, the authority on moral precepts, or rather a representation of the authority that is God who is the Law Giver, if you will. Is evil, evil because it is, or because God says that it is? By what standard do we make our judgement and is this judgement based in objectivity or subjectivity? The truth is that all morality is sourced from God, for without God morality is no longer objective and therefore is a facade.