Something which looks like a virtue, if followed, would be his (a leader's] ruin; whilst something else, which looks like a vice, yet followed brings him peace and prosperity. Machiavelli's advice can best be paraphrased as
1.Follow virtue, even if it leads to ruin.
2.Beware, vice can often look like virtue,
3.Take the action, whether virtuous or evil, that leads to peace and prosperity
4.Make decisions quickly, without taking too much time to think about them,

#1  sara was always tasked with selecting birthday presents for her friends and family because everyone admired her  impeccable taste. 

impeccable (adj): perfect, flawless

#2  they moved to the united states many years ago, but their  insular  attitudes made it difficult for them to socialize. 

insular (adj): isolated; separate

#3  the professor explained the  semantic  relationship between the two nouns used in the sentence. 

semantic (adj): related to words or the meanings of words 

happy to : )

because he was killed by the black arrow.