Any time you free range, you are taking a chance something like this will happen. I'm sorry it happened to you, but it is possible the rooster died defending his girls. If a predator (coyote, dog, fox, whatever) found a meal there, it will probably be back. There is a sticky at the start of the predator section on here that may help you identify what it was by the signs it left. I feel your others are in danger unless you either change the free ranging or deal with the predator. That is not always easy. Good luck.
If Artsyrobin's comment about the hen - rooster ratio bothers you, you might want to read this thread. I find that the personality of the hens and rooster are much more important than the hen - rooster ratio in determining how they are treated. This is another reason to get a more mature rooster since an adult is less likely to cause problems.
Breeders managing roosters
https://www.backyardchickens.com/forum/viewtopic.php?id=250327
The dynamics of adding a mature rooster to a flock of hens without a rooster are different than other integrations. They are living animals and have different personalities, so anything can happen. I find it is usually best to just turn a mature rooster loose with the hens. Usually, he will mate with them to establish his dominance, they accept him as head rooster, and things usually get peaceful fairly fast. Sometimes a hen will strongly resist the rooster, either trying very hard to resist his efforts to mate with her or actually challenging him for flock dominance. They usually work those issues out pretty quickly, but it can get violent for a while. Sometimes the rooster is a brute and does not deserve to live. As I said, anything can happen, but usually it goes pretty smoothly.
Quarantine is another issue. Many times people introduce a new chicken to their flock without any disease problems. Sometimes the flock gets wiped out. It is certainly a risk and certainly worth considering. Just quarantining the new chicken is not always effective. Sometimes chickens are carriers for diseases but are immune to the disease themselves. In these cases, quarantining the new chickens does absolutely no good. The symptoms will not show up no matter how long they are quarantined. I think coccidiosis is a good example, but there are others.
Some of the times where a quarantine is effective. If they have parasites, you can catch them and treat them before you introduce them to your flock. Many people treat any new birds for mites, lice, and worms before introducing them, whether symptoms are noticable or not. If the chickens have been around other new chickens in the last month, they may have been introduced to a new disease. An example would be where the chicken came from a chicken swap or a chicken show where it was exposed to many other new chickens. But if the chicken is coming from a flock that has not had any outside exposure to other chickens for a month or more, then a quarantine by itself will probably not do much good. Some exceptions to this are if the previous owner did not recognise the symptoms so did not even know that his chickens were infected or the previous owner is unethical enough to sell a diseased chicken. Wild birds can introduce a disease to a flock at any time. There is always a risk in not quarantining. I'm not arguing that. I take these issues serious enough that I personally do not add any living chicken to my flock. I get hatching eggs and hatch them to manage this risk.
I suggest a more effective means of quarantining is to put one of your existing chickens in with the new chickens. That way, if the new chickens have a disease they are immune to but can give to your flock, you are putting one of your chickens at risk instead of the whole flock. It is also possible that your existing flock has a disease that they are immune to and will infect the new chickens. If the new chickens come down with something and your old hen does not, this may be an indication. With three hens you may decide this is not worth the effort and you will accept the risk. That is purely a personal decision. We alll have different risk tolerances.
Good luck however you proceed.
