My experiences have shown me that usually stallions end up with their reputation through human interference and the way they are kept/handled.
Stallions are horses first and stallions second, BUT, being a stallion usually comes with an increase of dominance. This is often a non-issue until the day someone (or another horse) tries to dominate them - at which point it is not at all uncommon for their fight instinct to take over, and a stallion's fight instinct tells him he must fight until he wins, or die trying.
This is further impacted when they are not allowed to be a horse... Now they have instincts telling them to do something, but little or no learned experience to direct that instinct. This is when most stallions become the proverbial "dangerous animal". They do not know anything but that instinct which tells them to fight, or be killed trying.
In the wild, while there is one herd (breeding) stallion, there is also an array of "bachelors" who live on the outskirts of the main herd and co-exist, mostly, peacefully. This is generally because unless a younger stallion is born very dominant or another stallion from a different territory comes in - few will actually challenge the authority of the herd stallion. Generally a herd stallion can recognize this, and as long as he feels he is given the respect he is due, will allow those other stallions to live with the herd - as horses were meant to live. Generally fights are minimal, short lived and do not result in serious injury. There are exceptions though, especially when a stallion of equal or greater dominance comes along... Then it is the last man holding his ground - and that is where it has been noted that stallions will fight to the death. (When one or the other simply refuses to back down, they WILL keep going until it is decided by death)
Where this can impact domestic stallions in a herd turnout is when they either have never been raised to understand herd order (so a horse who is not naturally a born leader, but has never had to give ground to another horse - we will see social issues with mares or geldings like this too. These horses often become "bullies", or get the butt whooping of their life, as they do not know how to read the signs of a more alpha horse or those of a horse willing to submit to them... And things get carried away as the other individuals do what they feel they must), or when one is born naturally dominant and the human has failed to recognize this, inadvertantly turning him out with other alpha personalities (or tried to "teach him a lesson" by putting him with more alpha males... Which is often a recipe for disaster)
It also can impact a domesticated stallion during handling. It does not take much for a human to trigger that "fight or die trying" instinct. There is little difference between another stallion trying to aggressively dominate him or a human, especially to a stallion who has no idea about equine social order. Added to that fine line, is the fact that stallions kept in constant seclusion, aside from breeding time, are often already highly frustrated, making that thin line even thinner. A stallion constantly handled in fear, or constantly antagonized by his human handler is far more likely to lash out than one who has simply be handled in a way where he knows his boundaries. (And still, accidents have been known to happen, all it takes is for a hairy situation to come up and the person handling to be distracted for a few seconds)
What I have found common to ALL stallions is they are ready and willing to take over control of any situation, if they feel they need to... It takes a cool head and firm (but NEVER aggressive) hand to let them know they can relax and let the person make the decisions. Some stallions are more inclined to accept that than others, and young ones, almost always, are more ready than older ones (who already know they need not bother) to keep on asking if they need to take over. What often goes wrong, even with professional trainers, is the human crosses that line between "firm" and aggressive (usually when they got caught out, made an error, missed their timing etc. and are now in a position where they either need to make a "big statement" to the horse or "let the moment go" - most stallion handlers are taught to NEVER let the horse think he has the upper hand. This isn't all wrong, a stallion who thinks he is head honcho, even over humans, will be risky to handle at the best of times, but again, fine fine line... And where things will usually turn volitile.)
A stallion, to be as safe as possible to handle, needs to know, best early on, that people are not to fight, they are to trust and follow. This is most successful when the human is able to teach him to trust and follow in a consistent, firm, but not combative manner. This isn't to say you don't correct him, you do... And you do it with impeccable timing (if you can't time it right, you WILL have a problem... Timed right it is a correction, timed wrong you are being combative)... You also always need to be aware of your surroundings, and how they will affect his instincts - let's face it, he will NEVER fully trust you (and therefore be prone to those spontaneous actions which make many call stallions "dangerous") if you set him up for failure, or worse, let him down so he feels he MUST take over.
So no, those here who are coming across as "anti stallion" are not truly biased... There are some very real dangers to owning a stallion, especially if you never have before... They take a knowledge of equine behavior and natural equine instinct that very few people actually have, the best stallion owners and handlers not only have this knowledge, but they also combine it with the right energy and approach. Even then, accidents can still happen... People make mistakes (and so do even the best trained horses)