Unfortunately no vegetarian will go and kill others. That is why the earth is suffering.
One who lives by killing other living beings deserves to be killed, because by continuing such actions he is going to go down or to hell only. Killing him will save him from such sins.
Srimad Bhagavatham - Canto 1 - Chapter 7 - 37
sva-prāṇān yaḥ para-prāṇaiḥ
prapuṣṇāty aghṛṇaḥ khalaḥ
tad-vadhas tasya hi śreyo
yad-doṣād yāty adhaḥ pumān
SYNONYMS
sva-prāṇān—one’s own life; yaḥ—one who; para-prāṇaiḥ—at
the cost of others’ lives; prapuṣṇāti—maintains properly; aghṛṇaḥ—shameless;
khalaḥ—wretched; tat-vadhaḥ—killing of him; tasya—his; hi—certainly;
śreyaḥ—well-being; yat—by which; doṣāt—by the fault; yāti—goes;
adhaḥ—downwards; pumān—a person.
TRANSLATION
A cruel and wretched person who maintains his existence at the
cost of others’ lives deserves to be killed for his own well-being, otherwise
he will go down by his own actions.
http://vedabase.net/sb/1/7/en
Thirukkural also talks about this
JaiGurudev!