Does “slain” mean “dead” only?
No!

He is the Living Lamb who was Slain but Lives.

The Word is “slain” not “dead” here thus highlighting “pain”.

It’s written here that “Lamb Who was slain” not “Lamb who was dead”.

The verses which speak about the “Lamb being dead and lives” talk about the Triumph of His Resurrection.

Why?

Simple.

The verse says “slain from the foundation of the world” and thus it cannot mean “dead from the foundation of the world (lifeless)” since for example, when He walked the Earth, He was very much alive.

Thus, “slain from the foundation of the world” implies literally (without contradiction) that “pain from the foundation of the world”.

Death eventually Came when He died on the Cross but the act of being “slain” (in pain) is highlighted to be lasting for a very long time period (“from the foundation of the world”) and thus does not refer to His Death on His Cross only (which is inclusive in the time period of the ages in which He endures all pain of the world).

Further explanation.

He didn’t feel a “brief period of pain on the Cross” and “painlessness” elsewhere while His creatures suffer in pain due to sin.

He has felt and still feels all pain arising from all sin of the world until the World is Saved, comes to Pass as a fulfilled reality in time even into the ages to come.

Hence, He calls Himself not just the First but also the Last, in everything, e.g. here, that He becomes the Last to be relieved of pain too when the last creature to be restored is finally saved.

Truly, that’s why “God alone is Good”.