Been lurking here for a while and i though it would be interesting to adress this.
Shane has bitten the dust in season 2 and there was then various parallel in how they both try to handle same situation, some thinking Shane being more hardened than Rick, and that Rick would reach his level. We were being teased by some abiguous actions from Rick and his group in the following seasons, but it seems more blatant in season 5 and 6, that Rick and the group had crossed a lot of lines that made us reconsider their moral standards. Some might even consider them as nominal heroes instead of actual ones.
Let's recap Shane charges and compare them with Rick later actions.
Shane : Tried to get into a relationship with a married woman. (Lori)
Rick : My memory could serve me wrong, but it seemed to me that Rick tried to get into a relationship with Jessie while Pete was still in Alexandria.
Shane : Considered killing the husband of said woman. (Rick)
Rick : Although he was ordered to, Rick actually killed said husband (Pete). Also, he didn't hide he wanted to do it, before being ordered to.
Shane : Gave a beating to a violent husband (Ed)
Rick : Same, but with the intend to go further. (Pete)
Shane : Wanted to train a kid (Carl) to use guns, even if the parents weren't fond of it. Actually did.
Rick : Same with Ron. Carole also did it with Lizzie, which spectacularly backfired. (although she didn't have to wait that much for parent consent, considering their fate)
Shane : Considered killing his best friend (Rick), while it remains unclear if he would have the guts to actually do it.
Rick : Actually killed his best friend (Shane), but one could argue it was self-defense. (even if it remains quite ambiguous)
Shane : Wanted to drop the research for Sophia earlier than Rick.
Rick : I honestly don't remember if Rick or some other protagonist were reluctant to look for Beth. But i recall Rick & Carl being quick to consider Judith as dead after the final battle versus the governor.
Shane : Killing Randall, an unarmed enemy prisoner in cold blood. Advocating his execution beforehand.
Rick : Killed an unarmed Primo as one of a long list of unarmed antagonist, and nobody battled an eye. Also led of party to execute a lot of unawakened antagonists, although some were uncomfortable with it.
Shane : Killed an friendly ally (Otis) to ensure the sucess of his goal. Felt bad about it.
Rick : I don't think Rick even went that far. Altough, he chose to not help some people because he didn't want to take risk or didn't care about them. Some even died as a result. If we can't say they would have become friendly/efficient allies over time, they weren't identified as antagonist either. There is also the crazed man, another prisoner at Terminus, that i think was pushed agains't walkers to allow others to escape. Rick also executed some doomed characters (who were bitten) withouth batting an eye. Carole also did execute some allies, hoping for a better result for others. (like killing the sick in the prison, or killing Lizzie being safer than tries to help her change and risk others lifes)
Shane : Made implicit death threats agains't allies. (Dale)
Rick : Made explicit death threats agains't people that were aknowledged as not necessary antagonists (Aaron, amongs others)
Shane : Made a clear distinction amongs his allies between those he would risk everything for (Carl/Lori/maybe Judith & Andrea) and those he couldn't care less if they died.
Rick : Quite the same, especially at the Alexandria safe zone. Although, he tend to include much more people in the first group. On the other hand, it might be possible that, given time, Shane would have cared about more people.
Dunno if there is other things that could be blamed on Shane. I only watch each episode once.
We can also argue that Shane was quicker to take harsher solution, more short-tempered, and possibly more selfish than Rick. He also crossed a huge line for me when he killed Otis, although it allowed him to save Carl. (who knows what Rick would do to save Carl or Judith ?)
But in the end, one can't help to notice that a lot of what were considered amongs the darkest of Shane actions were greatly toned down by Rick & Co own actions in the later seasons. I mostly focused on Rick for the comparison as they were the competing leaders in season 2, but other Characters like Carole went even closer.