Here are some loose, early AM thoughts....
1. What were your first thoughts when you heard of John Lennon being murdered?
I was, at the time, not really into music as much as I had been growing up in the '60s, and nowhere near as much as now--it was a long period where I didn't do much more than listen to radio. I heard the news as I was getting ready for bed, and it was an immediate sense of personal loss, which surprised me as I hadn't thought of the Beatles or John Lennon all that much for 3 or 4 years. I recall a sense of sad nostalgia, regret that I had missed out on the last years of his life and art. In short, I think it struck me as a mixture of sadness and surprise that it affected me as much as it did.
2. What type of threat did Lennon present to Mark Chapman?
Well, from a John Lennon or fan's perspective, I'd say absolutely none. I don't know what was at work (or not at work) in Chapman's mind.
3. Lennon, as the celebrity he was, do you believe he put himself at risk to be harmed?
I don't think so...NYC is a city full of artists of various fame and repute. He'd been such a fixture there, I doubt he felt he was that much at risk, except for the hounding of the gov't.
4. Is there a cost to be paid for being successful or a celebrity?
I think obviously, yeah. Reading about their Beatlemania days, their lives were not their own to be sure, and it's reasonable to assume had JL become a nightclub performer with a "day" job it's unlikely that he would have been singled out by a deranged, obsessed individual, though I'm sure there are instances where this has perhaps occurred as well. I think to put it in the context of the times, from roughly the mid-'60s through the late '70s/early '80s, there were quite a few outspoken celebrities who drew negative attention for political positions, individual acts, etc., and yet none of them were murdered. John Lennon's murder, to me, was particularly tragic in that he really didn't represent a threat to anyone, if he ever really had. The act seemed to me to be aimed at destroying something of beauty, as opposed to an act of opposing views, etc..
5. Could Lennon be compared to Nelson Mandela, Mahatma Gandhi, and Martin Luther King?
Well, the wording in the question seems to pre-suppose the answer, but I'd disagree there's a legitimate comparison, or at least a strong comparison. The three individuals you offer as examples are known to us for their life-long opposition to oppression and social injustices--in other words, we know of them because of their struggles and sacrifice. John Lennon on the other hand was an artist we first met through his art, who then decided to use that fame and station in life we accorded him to try to generate social changes and awareness. Though their impacts and efforts eventually reached global proportions, the immediate efforts of the first three were substantially aimed at challenges in their immediate existence--Mandela and Apartheid, Gandhi at Colonial rule, and MLK at civil rights. Lennon's efforts, for the most part, seemed more global and less well-defined in nature (though he and Yoko did try and influence some specific causes).
Hope this is helpful to your project, just one man's opinion of course.
Cheers, Bill