I'm experiencing the same problem of Niels, also with your online demo at http://demos.telerik.com/kendo-ui/html5-diagram-sample-app
I'm implementing a diagram editor and I'm testing it with Firefox, Interner Explorer, Safari, Opera and Chrome
and only with Chrome the Diagram behave as I have the ctrl pressed.
Also selecting elements, for example, I click on them and they are selected as ctrl was pressed.
(Windows 8.1 + Chrome 47.0)