Hamlet's arguments for not killing Claudius at prayers are both subtle and logical -- too subtle, in fact, considering the enormity of Claudius' deed and the virtual certainty that Hamlet possesses of his guilt. Hamlet was a weak character, which caused him to procrastinate.
Hamlet: Do you think I meant country matters? Hamlet and Fortinbras both have plans that are thwarted by uncles that are also kings. Our first impression of Hamlet sets the tone for the whole play.
Both are rather spineless and obedient—Rosencrantz moreso than Guildenstern—and neither is intelligent enough to really fool Hamlet. Ophelia: I think nothing, my lord.
Ophelia, like her father, waits in vain for Hamlet to give her signs of affection, and Horatio would have little reason to think that Hamlet was concerned with anything more pressing than the commandment of the ghost.
The characters are deeply suspicious of one another, as it becomes clear that the king may have been murdered—and by his brother Claudius no less.