@cerealgamechanger
I'm cautious to pass judgement and require more evidence because history has generally been very kind to Ranjit Singh and he is considered to have been a good example of what a good leader should be like (This was being circulated on Twitter only last year Who is the greatest leader in world history?).
Regarding the Kohinoor, history was written by the victors who cynically exploited Ranjit Singh's child son Duleep; it was in British interests to suggest that Ranjit Singh had acquired the diamond through torture to make it look ok for how they themselves acquired the diamond. Even if Ranjit Singh did acquire the Kohinoor diamond through torture, then two wrongs don't make a right. Truth is, we may never know the truth...
There's a movie on Netflix called 'The Black Prince' about what happened to Ranjit Singh's son, Duleep.
What happened there seems to be more of an injustice than taking the jewel itself imo. Duleep converted to Christianity, so had a good grasp of what British values in Victorian times were. However, when his mum had explained the other side of the story later on in life she basically said, "the British Empire are not the good guys, look it up." After she died, Duleep read up about what happened to him as a child in the British Library and became very angry about the injustice because it was also blatant hypocrisy from Victorian Christian Britain. Queen Victoria was acutely aware of the advantage that had been taken of Duleep, which is why she befriended him.
Not much is written about what happened and it seems a lot of British people don't care.
Personally, I find it hard to reconcile how the current Queen's Mother could have worn a crown adorned with the Kohinoor as its main diamond and not perceive the loaded moral issues she was carrying on her own head. 
It is a charged issue. If the British Royal Family never gave the diamond back to the Duleep Singh family who it was taken from in the first place (Duleep did ask for it back), then it's unlikely they will be wilfully giving it to anyone else. The attitude is more like, "We have it and that's that", but I suspect it's the risk of humiliation and an admission of "we stole this, here it is back." Not a good look for the Defender of the Faith, is it?
Pretty sure Old Testament God has a few commandments and one of those is "Thou Shalt Not Steal". Sound commandment even if you don't believe in God.
Britain could return the Kohinoor, but they can't return the lost/ended royal bloodlines.