If the relationship is genuinely over and there is no contact between you, visiting another inmate is your personal choice and not something that requires anyone else's approval.
Respect and disrespect in this context are entirely subjective. What one person considers a betrayal another considers a completely reasonable exercise of personal freedom, particularly after a relationship has ended. The question worth sitting with is not whether your ex would approve, because that approval is not yours to seek once a relationship is done, but whether your own motivations and intentions feel right to you.
The honest answer to the underlying question is that society and prison culture both tend to have strong opinions about loyalty and perceived slights, and those opinions are not always reasonable or fair. People judge, particularly in environments as tight and socially charged as correctional facilities where information travels fast and perceptions matter in ways they do not on the outside.
If the relationship is truly over, you are free to visit whoever you choose. The more relevant considerations are practical ones. Are you going to the same facility where your ex is housed? If so, that creates a situation worth thinking through carefully, not because you owe your ex anything, but because visiting rooms are shared spaces and the potential for awkwardness or conflict is real.
Make decisions based on what is right for your own life rather than on managing the feelings of someone you are no longer with. You already know the answer to the golden rule question this situation raises. Let that guide you.