One question bugs me. Gandhiji played the pivotal role in India's Freedom Struggle. He became a pain for the British early on. For the British to put a single bullet into an aging man was not much of a challenge. Why did they not do it ? Why did they keep Gandhiji alive ?

I know the topic is sensitive. But still it bugs me.

In the beginning Gandhi was very much supportive of English ways, he supported british in many wars like World war 1, Boer war, war against zulus and even recruited Indians to join in the war. So, for britishers, he was not a foe to be feared.

Its after some tragedies like Jalianawala Bagh, land tax, salt tax, rejection of Dominion status to India and other incidents he started opposing britishers vehemently.

But by that time his popularity was so high that it would be impossible to kill because of fear of mass unrest and chaos.

Mind it - There were many attempts on Gandhi to kill him but he was lucky.

Its rare in progressive democratic political system to kill the leaders. They may be confined or put behind bars.

We can see many examples like that
Prachanda, Suu kyi, Nelson Mandela, Yasser arafat etc.

"There were many attempts on Gandhi to kill him but he was lucky." please elaborate on this part.

In fact there is quite a controversy regarding his stance on non-violence principles for his support to British wars.

Here refer this quote from Wikipedia:

In the summer of 1934, three unsuccessful attempts were made on Gandhi's life.

Here is the link where you can find the above quote

Mahatma Gandhi