It's important to remember also that cockers are a VERY popular breed - they are usually up in the top 3 most produced dogs every year, so whilst there are lots of issues on the behaviour board here, it doesn't reflect that behavioural issues are more common in cockers nor does it suggest that every cocker will have them. It's likely you will have some undesirable behaviour at some point as that's true of nearly all puppies of all breeds, but managing it and working positively to train your dog is what's needed!
It's also important to remember why cockers exist, they were originally bred as working dogs, one of their biggest jobs being to track down and 'flush' birds out of the undergrowth. For this skill, busy, energetic, intelligent dogs were selected that had the stamina to 'work' all day if required and these traits are still very present in todays cockers (including a lot of show types) even though most cockers are now 'just' pets and don't do the work they were bred to do. So this energy, intelligence and drive needs to be channelled otherwise dogs can become bored and frustrated which can lead to behavioral problems. Cockers need mental stimulation (training and games etc) and once they are fully grown, a good level of exercise for them to be content... without this, it's more likely that you will have behavioral issues - but as already said, that's nothing to do with rage.
I train my dogs every day on walks - it's no big deal, but we don't just walk. I keep them busy with obedience training and some gundog type games that keep their brains working and occupied, they love it and are happy, responsive and generally well behaved because of it.
My Weimaraner (a rescue) can be aggressive (dog to dog, not human) because she was undersocialised as a pup and this is helped by lots training and building a trusting bond.
You won't get a readymade 'good' dog with any pup, but you will have a busy cocker pup that's full of life and needs to be entertained, stimulated and exercised to be a happy well balanced dog.
