Revelation 6:11; Until

Revelation 6:11 And white robes were given unto every one of them; and it was said unto them, that they should rest yet for a little season, until their fellowservants also and their brethren, that should be killed as they were, should be fulfilled. 

Swindoll observed well, “After watching the four horsemen gallop across the darkening landscape, the apostle John witnessed the Lamb breaking the fifth seal (v9-11). This marks the next event of the Tribulation, providing us with some information regarding believers during the Tribulation. The church, having been rescued before the Tribulation by being caught up to heaven at the Rapture, will already have been rewarded in their glorified bodies (3:10). However, those who are left behind and come to faith in Christ during the seven-year Tribulation period will face the greatest persecution the world has ever known (Matthew 24:21). These martyrs, who suffer at the hands of the Antichrist described in Revelation 13, will ultimately be vindicated when God pours out His wrath on His enemies.

The fact that these souls are “underneath the altar” (v9) indicates that their untimely deaths will be valiant martyrdoms, lives given for the sake of the gospel and in the service of their Savior (v9). During this time, those who violently depart this life will be in a bodiless state awaiting resurrection. 
[Swindoll’s Living Insights New Testament Commentary – Revelation]

Daniel 12:2 And many of them that sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt. 

Daniel 12:3 And they that be wise shall shine as the brightness of the firmament; and they that turn many to righteousness as the stars for ever and ever. 

No regrets for these who loved the Lord even unto death. Amen.