I voted "Other, please explain", because:
Romans 12 - we must not avenge ourselves:
17 Repay no one evil for evil. Provide things honest in the sight of all men.
18 If it is possible, as far as is in you, seeking peace with all men.
19 not avenging yourselves, beloved, but giving place to wrath; for it is written, "Vengeance is Mine, I will repay, says the Lord."
20 Therefore if your enemy hungers, feed him. If he thirsts, give him drink. For in so doing you shall heap coals of fire on his head.
21 Do not be overcome by evil, but overcome evil with good.
2 Thessalonians 1-God will avenge us:
6 since it is a righteous thing with God to repay tribulation to those who trouble you,
7 and to give rest with us to you who are troubled, at the revealing of the Lord Jesus from Heaven with the angels of His power,
8 in flaming fire taking vengeance on those who do not know God and who do not obey the gospel of our Lord Jesus Christ,
So the question of the martyrs under the altar must be understood in the light of the above scriptures. They know God is going to, and will avenge their blood, so they're asking how long it will be before God does so. Nothing wrong with their question.