There are three accountabilities in a Scrum Team, a Scrum Master, a Product Owner and the Developers. Each is accountable for one or more different parts of delivering a useful product. They are dependent on each other. Unless they all fulfil their accountabilities effectively, the product is at risk.
Consider the accountabilities as a three-legged stool – simplified to Process (Scrum Master), Quality (Developers) and Value (Product Owner). The product is sat on and supported by the stool. Each leg is of equal length and all are needed together. In Scrum, there is no hierarchy between accountabilities as each is vital to successful and frequent delivery of the product. If one leg breaks, the product falls down. With this metaphor, the three legs are not likened to people holding roles but rather actions fulfilled by unspecified members. Each leg could be supported by anyone as part of a self-managing and committed Scrum Team.
To evidence this point, let’s take a look at a case-study…from 2,500 years ago. The Battle of Cynoscephalae  was an epic engagement between the Roman and Macedonian armies led by Flamininus and Philip V respectively. Each army was relatively balanced in terms of power and resourcing, but they were organised differently. The Romans used a manipular system  (small units of ‘brothers in arms’, with a range of experiences and styles) compared to the Macedonian phalanx system (a strong, rigid formation based on mass and uniformity). The Roman maniples were each empowered to make decisions on the battlefield without orders – they were trusted to get the job done, and produce the value seen through victory. To liken them to Scrum, they had a Product Owner (Tribune), Developers (Hastati and others) and Scrum Master (Signifer) – they had no direct commander.
Midway through the battle, the Tribune noticed that the left flank of the Macedonians was open (a gap in the market he could target) and without receiving any orders, he asked the maniple to pivot and exploit it. Suffice it to say, the ability of this small, empowered maniple won the Romans a decisive victory. The Tribune paid attention to the evidence he was observing and decided to adapt appropriately. Having the ability to innovate and rapidly change course depending on market conditions is why an effective Scrum Team will be self-managing. Why should a Commander (external manager) be expected to make decisions without seeing the evidence on the ground? The Roman Commander, Flamininius, understood this, and whilst he helped shape the maniples, skill them and provide an overall direction, he trusted them to deliver.
The accountabilities in Scrum are powerful when understood and allowed to thrive. They support each other as a single collective (Scrum Team) and execute as a unit. Importantly, using the word ‘role’ should not be considered wrong. It’s about context – you may be hired for a role (consider whether an archer could easily transition to be a swordsman?) but that doesn’t change what you and your team are accountable for achieving (winning the battle). Everyone has a different part to play, but when the game is over you stand together as a team, not as individuals. There is a huge difference between ‘doing a role’ and ‘fulfilling an accountability’.