Four Regulative principles

Hare Krishna!
a. No Meat eating. Meat eating destroys the mercy quality.
b. No Illicit Sex. Illicit sexual connections destroys cleanliness.
c. No Intoxication. Intoxication impedes the ability to perform austerities.
d. No Gambling. Gambling destroys truthfulness.

Is killing plants for our food sinful? Yes. 

However Supreme Personality of Godhead Krsna eradicates those sins, if food is offered to Him with love and devotion. Reference: Bhagavad Gita Verse 9.26. "If one offers Me with love and devotion a leaf, a flower, fruit, water, I will accept it". The food becomes sin free, if one offers the food to Krsna with love and devotion.