Apple has setup up a number of defaults behaviors especially in the latest versions of iOS that removes "Delete" from your list of actions that you can do with email from the default iOS mail. Here are the steps to add that power back. As with any power, use wisely.
1. First, tap open “Mail, Contacts, Calendars” in Settings and choose the account(s) you want to affect.
2. In that account’s settings, tap on “Account”.
3. Below Outgoing Mail Server, you will see an “Advanced” option, tap on that.
4. Now, simply check “Deleted Mailbox” under the Move Discarded Message Into heading.
5. Finally, make sure you go back to the account settings screen and tap “Done” in the upper-right corner so the new changes stick.
Now the default swipe behavior will be to delete messages rather than archive them.