Was Arrow's change in moral alignment motivated by his audience?

I haven't read anything official that would point to this, but I do believe that the shift in Oliver's moral convictions was planned from the start.

In the first season, Oliver's alter ego was called The Hood. The Hood had no qualms in killing people he thought deserved it. Come the second season and Oliver has a change of heart. One of the first things he does is change the name of his alter ego to The Arrow (while slyly looking at a green one to appease us comic fans!). That's the name of the series. So that tells me that the writers had planned a shift, in naming at least, from the inception of the show.

I think @System Down has summed up all the imported points. Just to add something though, here is an interview from Time Magazine the shows co-creator, Mark Guggenheim, where he discussed the change of direction in Season 2 compared to Season 1:

TIME: Were there different sorts of things you wanted to accomplish with Season 2 compared with Season 1?

Marc Guggenheim: Yeah, great question. Well, our goal, you know, sort of on a meta-level was our goal is to top ourselves from season 1. I think that’s probably fairly obvious, or that’s an obvious call that I think any second season show should have. We went in, we knew we wanted to tell a very concrete story with respect to Oliver making a journey from vigilante to hero. And we knew we wanted to center the evolution around this idea of him giving up killing as a means of accomplishing his ends. So we knew we had that sort of core dynamic to play with.

So this provides the reason why they made Oliver kill less - to show him as a true hero, rather than a vigilante. However, late in the same interview he mentions that the decision to kill Tommy was only made around episode 17/18 in the first season - the actor was actually on a long term contract and they made a decision to honour the theme of sacrifice throughout the first season by killing him to give everyone an emotional jolt. I mention this because it shows that the show's creators were very fluid in their decisions and were happy to make impromptu calls in the interests of the themes of the show.

Based on that fact, I don't know if it can conclusively be said that they always wanted Oliver to kill less in his transition to becoming a hero. However, given how Marc Guggenheim has discussed it in the quote above, I would say that that interpretation is likely.

While the show definitely changed a lot of the details about the Green Arrow, it seems to be based off the new comic series ( "New 52" or "New Earth" I believe). In the comics Oliver, undergoes a similar change, so I believed the writers always planned the shows change from season 1 into 2.

Here's My Opinion:

First: know that in TV series usually nothing is obvious from the begining, USUALLY, as for the CW series Supernatural, I have heard producers were only making 4 seasons, but they continued when they saw the unexpected number of fans and followers.

and

Second: there are so many alterations to characters, like Slade Wilson and Oliver's background on the island, and practically half of Batman's main enemies on the show, altering Talia al Ghul to Nyssa Raatko, and adding some points of consciounce to characters like Deadshot. So I think this was meant to be Oliver's alteration, and that he was not who he is from the begining, and also this is his weakness as he constantly changes tactics and underestimates his colleages capabilities.

In the end, every movie or series adaptation has it's own alterations from the original source, and remember the fact that Oliver was trained by Slade, so in code of military operations you rather kill instead of taking prisoners, and the fact that things he did for ARGUS after the Island changed him and his methods further. I haven't read a single DC Comic, and just saw Nolan's Batman trilogy and a couple of other Batmans before that, and the Batman Arkham games, and in between I think the games would have been more accurate to the roots.