Why is the Lamb described as “standing, as though it had been slain”?

Revelation 5:6 (ESV)

6 And between the throne and the four living creatures and among the elders I saw a Lamb standing, as though it had been slain, with seven horns and with seven eyes, which are the seven spirits of God sent out into all the earth.

A slain lamb obviously does not stand. Its throat is slit, its blood shed; it collapses from weakness and dies. This lamb was standing even though its throat was slit (and the Greek indicates that it remains standing). That’s because this Lamb was slain as an atoning sacrifice for sin, then raised from the dead to demonstrate that its sacrifice was acceptable to God, then taken into heaven at the ascension so that it is now standing before the throne victoriously.