Illah and Toviah: [were] witnesses on a loan and they were related to the guarantor.

If we were: around in the time that court would rule in capital cases no one would ever get executed as the gemara explains that they would cross-examine the witnesses in matters that they would not know how to answer.

Even: If you would have done this you would have increased murder in Isreal because people would no longer fear court.

MISHNAH: One Was Roofing With a Roller - They used to plaster their roofs with clay. [The roofs] were flat but the clay would give them a slight incline so that the [rain] water would run off. They would plaster it with a wide, smooth piece of wood that had a handle which they would push towards the incline and then draw it back towards himself, and then push it again etc. causing the clay to smear and smoothen. The pushing movement is referred to as "מעגל" - "ma'agal", and the drawing motion is referred to as "מושך" - "moshech". The wooden [tool] is referred to as a "מעגילה" - "ma'agilah" [a trowel].

He was lowering a barrel: from the roof.

If He Was Descending - [If ] the killer [was descending] the ladder and then fell off it killing [the victim] with [the force of] his body, he is exiled. All of these cases are [examples of] downward motion, which is what is required in order to obligate exile, as the Gemara derives [from the words of the verse] "and he caused to fall upon him."

However, If He Was Pulling - and the trowel slipped from his hand and fell.