I'm not talking about making friends. I'm saying that an army is made of people, people have emotions, emotions color judgement and that can lead to mass desertions if the leader gets called into question.
Part of being effective is being able to inspire confidence in the people you lead. And if they start to think that they have a better chance without you, well... stuff happens.
Given that wannabe ruthless tend to like quoting him, here's something from Machiaveli:
"Upon this a question arises: whether it be better to be loved than feared or feared than loved? It may be answered that one should wish to be both, but, because it is difficult to unite them in one person, it is much safer to be feared than loved, when, of the two, either must be dispensed with. Because this is to be asserted in general of men, that they are ungrateful, fickle, false, cowardly, covetous, and as long as you succeed they are yours entirely; they will offer you their blood, property, life, and children, as is said above, when the need is far distant; but when it approaches they turn against you. And that prince who, relying entirely on their promises, has neglected other precautions, is ruined; because friendships that are obtained by payments, and not by greatness or nobility of mind, may indeed be earned, but they are not secured, and in time of need cannot be relied upon; and men have less scruple in offending one who is beloved than one who is feared, for love is preserved by the link of obligation which, owing to the baseness of men, is broken at every opportunity for their advantage; but fear preserves you by a dread of punishment which never fails.
Nevertheless a prince ought to inspire fear in such a way that, if he does not win love, he avoids hatred; because he can endure very well being feared whilst he is not hated, which will always be as long as he abstains from the property of his citizens and subjects and from their women"
Fear is a powerful tool for the prince/ruler/boss/whatever. But hate? Hate is much more powerful, and impossible to control. It can devour even the most fearsome prince. Machiaveli knew that. He also pointed out that the best of the best would be being feared and loved at the same time, but cynical as he was, he deemed it difficult (but not impossible). The ruthless forget that, and the part about hate too.