Regulus knew it wasn't about him; it never had been- he was no where near important enough for his brother to make a life altering decision on his behalf. Otherwise he would have stayed. No, Regulus berated himself, that isn't fair of him to think. It wasn't that Sirius didn't care about Regulus, it's just that he never considered the consequences of his actions. He never considered that having half as many children left meant their parents would inflict twice as much pain on the remaining one. He never considered that leaving meant Regulus would be their parents only option for an heir; that this meant it would be thrice as hard for him to leave. He never considered that the only reason their parents allowed Sirius to leave without major repercussions was that they still had what they needed in Regulus. He had wanted so desperately to take Sirius' hand, to escape this hell together, but he knew that their parents would never allow the both of them to leave- not when they had yet to fulfil the one purpose they existed for. So, Sirius had left Regulus to burn alone. Without another source of kindling, the flames quickly consumed Regulus in his entirety.
Regulus could hardly blame his brother though. In the end, all elements of his suffering had one thing in common- that the cause was his parents. If Sirius had stayed, like Regulus had idiotically believed he might in the second before he dove into the floo flames, then he would have died. Regulus might not be good at the social related aspects of life but he was perceptive, and he prided himself on that, so it hadn't escaped his notice that Sirius was careening towards insanity as a result of this house and the goings on inside it. This was how he knew for sure that Sirius would have died had he stayed, maybe not his body- for that still had use to his parents- but his soul and perhaps his mind.
The same could be said for all the members of the noble and most ancient house of Black, he supposed. The lucky few escaped, and the ones left behind go mad. Some in a more obvious way, like his cousin Bellatrix, others in a more subtle way- such as marrying inside the family. Insanity was a requirement for those who remain, after all anyone in their right mind would leave.
Regulus had been of stable mind at one point, before Sirius had left him trapped in this house, in this family. Now, he was clearly a proper part of the Black family- insane, Slytherin, Death Eater and about to commit an act of uttermost backstabbing. Although, usually the backstabbing was done outside of the family, unlike the breeding. Maybe Regulus was more like Sirius than he thought, certainly not in bravery but in stupidity. On second thought, Regulus pondered, only James Potter could compete with his brother in the stupidity department; it was a good job Potter was so attractive or he would never have achieved his popularity status.
Luckily for Regulus, he would be dead soon and therefore would not have to acknowledge these pesky feelings that James Potter's infernal attractiveness and warm-hearted tendencies caused in him.
He hoped this would kill him anyway, if it didn't then he would have to deal with the noseless bastard's whining at the perceived betrayal. Regulus didn't really consider it as such, seeing as he had never felt any loyalty to the tyrant, but he doubted that explanation would garner any sympathy.
Maybe he should have informed someone of his findings. The horcrux's would be crucial in taking down Voldemort and Regulus didn't know if anyone else was actually aware of their existence, but telling others meant putting them in harms way. Sure, anyone he could tell was already involved in the war, but it would be the difference between death being a likelihood or a certainty and that wasn't a risk he was willing to take. Additionally, they would all feel the need to stop Regulus from doing what he was about to do, and that would be terribly bothersome. So, despite not wanting to die alone in a dingy cave, Regulus was about to do so. Better him than his friends, or his brother.
Sirius would likely be devastated when he got the news, but he had good friends and a good life now. He would work himself back to full happiness speedily enough, before he knew it the devastation would turn into the occasion pang of longing for the brother he once had- or perhaps a sting of guilt at believing his little brother had turned to the dark side when the kid he had known would never have done such a thing. Regulus didn't blame his brother for thinking he had followed in his parents footsteps, that was the role he played and he was just egotistical enough to know he had played it well.
The locket in his hand was heavy. The feeling in his chest wasn't regret- this was something that must be done and it was best that he was the one to do it- it was remorse. Remorse for any misplaced guilt or pain felt as a result of his removal from the mortal plain. Remorse for the life he could have lived, both if he had been born to a different family and despite having the misfortune of being born to this one.
Oh well, he sighed internally, sometimes people just aren't meant for greatness. Sometimes humans are put on the earth just to be cogs in the larger machine. Regulus didn't deserve to be hero for the simple act of being born, he didn't deserve glory for the admittedly impressive feat of surviving this long and he certainly wouldn't earn anything for dealing with his family or for making this sacrifice. That's the way of life. Everything must be balanced, his suffering was vital in balancing out the enjoyment of some stranger, his pain ensured some one else's peace and now his death would guarantee another life would not be taken so early. Sacrifices worked in this way, he supposed, Regulus would just have to give his life and hope it would be his brother, his friends and his crush who would benefit from it.
The water dragged him under, pulling him by the weight of his heavy clothes and heavier heart. Regulus' throat burned and his lungs ached. Instinct would always push for survival, evident in the way his whole body screamed for oxygen, but to survive is to suffer and to die is to give life so, for the last time, Regulus fought against his instincts. Allowing himself to be suffocated, welcoming the darkness that evaded his vision.
Life had been cruel, but Regulus hoped death would be fair.