He didn't say that it's reasonable to kill 1800 people (+/-) in exchange for 3000 rockets. But Israel didn't receive rockets and say "let's go kill a couple thousand people". That would be unreasonable.
What isn't unreasonable is to say "we're going to attack the sites that attack us" and that tunnels will be destroys... and that searched buildings with explosive booby-traps will be triggered... and that attacks in-country will meet retaliation. (etc etc etc). Civilian deaths may very well occur (not that Hamas has ever cared about that)... but isn't the intent.
If they do what they can to reduce civilian casualties where possible (without sacrificing legitimate military targets) and Hamas doesn't do that (indeed... goes out of their way to endanger those civilians)... then lots of civilian deaths will naturally result.
Do you, or do you not think it was reasonable for Israel to carry out an offensive that resulted in the deaths of 1,800, mainly civilians?
It was very reasonable for Israel to carry out an offensive. It wasn't reasonable for Hamas to endanger so many of the people they were obligated to protect.